create copy of read only object

Author: jigfox

WKWebView.ios.js

@@ -312,7 +312,7 @@ class WKWebView extends React.Component {
       WKWebViewManager.startLoadWithResult(!!shouldStart, event.nativeEvent.lockIdentifier);
     });
 
-    let source = this.props.source || {};
+    let source = Object.assign({}, this.props.source || {});
     if (typeof source == 'object') {
       source.sendCookies = this.props.sendCookies;
       source.customUserAgent =