Fixes loadAfterBeforeload on iOS. Resolves #349. (#350)

For both UIWebView and WKWebView implementations on iOS.

<!--
Please make sure the checklist boxes are all checked before submitting the PR. The checklist
is intended as a quick reference, for complete details please see our Contributor Guidelines:

http://cordova.apache.org/contribute/contribute_guidelines.html

Thanks!
-->

### Platforms affected
iOS

### What does this PR do?
Fixes `beforeload` event (introduced by #276) for iOS

### What testing has been done on this change?
Tested both allow & deny loading of URL with both iOS implementations in [test container app](https://github.com/dpa99c/cordova-plugin-inappbrowser-wkwebview-test) (ignore its README).

- To test with UIWebView use options: `beforeload=yes,usewkwebview=no`
- To test with WKWebView use options: `beforeload=yes,usewkwebview=yes`

### Checklist
- [x] [Reported an issue](http://cordova.apache.org/contribute/issues.html) in the JIRA database
- [x] Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [x] Added automated test coverage as appropriate for this change.

closes #349

src/ios/CDVInAppBrowser.h

@@ -30,6 +30,7 @@
 - (void)injectScriptCode:(CDVInvokedUrlCommand*)command;
 - (void)show:(CDVInvokedUrlCommand*)command;
 - (void)hide:(CDVInvokedUrlCommand*)command;
+- (void)loadAfterBeforeload:(CDVInvokedUrlCommand*)command;
 
 @end
 

src/ios/CDVInAppBrowser.m

@@ -121,5 +121,14 @@
     }
 }
 
+- (void)loadAfterBeforeload:(CDVInvokedUrlCommand*)command
+{
+    if(self.usewkwebview){
+        [[CDVWKInAppBrowser getInstance] loadAfterBeforeload:command];
+    }else{
+        [[CDVUIInAppBrowser getInstance] loadAfterBeforeload:command];
+    }
+}
+
 
 @end

src/ios/CDVWKInAppBrowser.h

@@ -27,6 +27,9 @@
 @class CDVWKInAppBrowserViewController;
 
 @interface CDVWKInAppBrowser : CDVPlugin {
+    @private
+    BOOL _useBeforeload;
+    BOOL _waitForBeforeload;
 }
 
 @property (nonatomic, retain) CDVWKInAppBrowser* instance;
@@ -40,6 +43,7 @@
 - (void)injectScriptCode:(CDVInvokedUrlCommand*)command;
 - (void)show:(CDVInvokedUrlCommand*)command;
 - (void)hide:(CDVInvokedUrlCommand*)command;
+- (void)loadAfterBeforeload:(CDVInvokedUrlCommand*)command;
 
 @end
 

src/ios/CDVWKInAppBrowser.m

@@ -60,6 +60,8 @@ static CDVWKInAppBrowser* instance = nil;
     instance = self;
     _previousStatusBarStyle = -1;
     _callbackIdPattern = nil;
+    _useBeforeload = NO;
+    _waitForBeforeload = NO;
 }
 
 - (id)settingForKey:(NSString*)key
@@ -263,6 +265,9 @@ static CDVWKInAppBrowser* instance = nil;
         }
     }
     
+    // use of beforeload event
+    _useBeforeload = browserOptions.beforeload;
+    _waitForBeforeload = browserOptions.beforeload;
     
     [self.inAppBrowserViewController navigateTo:url];
     [self show:nil withNoAnimate:browserOptions.hidden];
@@ -370,6 +375,27 @@ static CDVWKInAppBrowser* instance = nil;
     [[UIApplication sharedApplication] openURL:url];
 }
 
+- (void)loadAfterBeforeload:(CDVInvokedUrlCommand*)command
+{
+    NSString* urlStr = [command argumentAtIndex:0];
+
+    if (!_useBeforeload) {
+        NSLog(@"unexpected loadAfterBeforeload called without feature beforeload=yes");
+    }
+    if (self.inAppBrowserViewController == nil) {
+        NSLog(@"Tried to invoke loadAfterBeforeload on IAB after it was closed.");
+        return;
+    }
+    if (urlStr == nil) {
+        NSLog(@"loadAfterBeforeload called with nil argument, ignoring.");
+        return;
+    }
+
+    NSURL* url = [NSURL URLWithString:urlStr];
+    _waitForBeforeload = NO;
+    [self.inAppBrowserViewController navigateTo:url];
+}
+
 // This is a helper method for the inject{Script|Style}{Code|File} API calls, which
 // provides a consistent method for injecting JavaScript code into the document.
 //
@@ -480,16 +506,29 @@ static CDVWKInAppBrowser* instance = nil;
  * to the InAppBrowser plugin. Care has been taken that other callbacks cannot be triggered, and that no
  * other code execution is possible.
  */
-- (BOOL)webView:(WKWebView*)theWebView decidePolicyForNavigationAction:(NSURLRequest*)request
-{
-    NSURL* url = request.URL;
-    BOOL isTopLevelNavigation = [request.URL isEqual:[request mainDocumentURL]];
+- (void)webView:(WKWebView *)theWebView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler {
+    
+    NSURL* url = navigationAction.request.URL;
+    NSURL* mainDocumentURL = navigationAction.request.mainDocumentURL;
+    BOOL isTopLevelNavigation = [url isEqual:mainDocumentURL];
+    BOOL shouldStart = YES;
+
+    // When beforeload=yes, on first URL change, initiate JS callback. Only after the beforeload event, continue.
+    if (_waitForBeforeload && isTopLevelNavigation) {
+        CDVPluginResult* pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK
+                                                      messageAsDictionary:@{@"type":@"beforeload", @"url":[url absoluteString]}];
+        [pluginResult setKeepCallback:[NSNumber numberWithBool:YES]];
+
+        [self.commandDelegate sendPluginResult:pluginResult callbackId:self.callbackId];
+        decisionHandler(WKNavigationActionPolicyCancel);
+        return;
+    }
     
     //if is an app store link, let the system handle it, otherwise it fails to load it
     if ([[ url scheme] isEqualToString:@"itms-appss"] || [[ url scheme] isEqualToString:@"itms-apps"]) {
         [theWebView stopLoading];
         [self openInSystem:url];
-        return NO;
+        shouldStart = NO;
     }
     else if ((self.callbackId != nil) && isTopLevelNavigation) {
         // Send a loadstart event for each top-level navigation (includes redirects).
@@ -499,8 +538,16 @@ static CDVWKInAppBrowser* instance = nil;
         
         [self.commandDelegate sendPluginResult:pluginResult callbackId:self.callbackId];
     }
+
+    if (_useBeforeload && isTopLevelNavigation) {
+        _waitForBeforeload = YES;
+    }
     
-    return YES;
+    if(shouldStart){
+        decisionHandler(WKNavigationActionPolicyAllow);
+    }else{
+        decisionHandler(WKNavigationActionPolicyCancel);
+    }
 }
 
 #pragma mark WKScriptMessageHandler delegate
@@ -1072,10 +1119,7 @@ BOOL isExiting = FALSE;
         self.currentURL = url;
     }
     
-    [self.navigationDelegate webView:theWebView decidePolicyForNavigationAction:navigationAction.request];
-    
-    decisionHandler(WKNavigationActionPolicyAllow);
-    
+    [self.navigationDelegate webView:theWebView decidePolicyForNavigationAction:navigationAction decisionHandler:decisionHandler];
 }
 
 - (void)webView:(WKWebView *)theWebView didFinishNavigation:(WKNavigation *)navigation