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