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

For both UIWebView and WKWebView implementations on iOS.

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### 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.m

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