fix(iOS): use RCTConvert to convert allowingReadAccessToURL. (#845)

* fix(ios):use RCTConvert to convert url instead of using URLWithStringg

* fix(iOS):remove stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding, because it's deprecated in iOS 9

ios/RNCWebView.m

         [_webView loadRequest:request];
     }
     else {
-        NSURL* readAccessUrl = _allowingReadAccessToURL ? [NSURL URLWithString:_allowingReadAccessToURL] : request.URL;
+        NSURL* readAccessUrl = _allowingReadAccessToURL ? [RCTConvert NSURL:_allowingReadAccessToURL] : request.URL;
         [_webView loadFileURL:request.URL allowingReadAccessToURL:readAccessUrl];
     }
 }