feat(WKWebview): Add 'userAgent' property to the iOS WKWebView. (#112)

* Add 'userAgent' property to the iOS WKWebView

* Update 'userAgent' reference docs.

docs/Reference.md

 
 ### `userAgent`
 
-Sets the user-agent for the `WebView`.
+Sets the user-agent for the `WebView`. This will only work for iOS if you are using WKWebView, not UIWebView (see https://developer.apple.com/documentation/webkit/wkwebview/1414950-customuseragent).
 
 | Type   | Required | Platform |
 | ------ | -------- | -------- |
-| string | No       | Android  |
+| string | No       | Android, iOS WKWebView  |
 
 ---
 

ios/RNCWKWebView.h

 @property (nonatomic, assign) BOOL automaticallyAdjustContentInsets;
 @property (nonatomic, assign) BOOL hideKeyboardAccessoryView;
 @property (nonatomic, assign) BOOL allowsBackForwardNavigationGestures;
+@property (nonatomic, copy) NSString *userAgent;
 
 - (void)postMessage:(NSString *)message;
 - (void)injectJavaScript:(NSString *)script;

ios/RNCWKWebView.m

     _webView.scrollView.bounces = _bounces;
     [_webView addObserver:self forKeyPath:@"estimatedProgress" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:nil];
     _webView.allowsBackForwardNavigationGestures = _allowsBackForwardNavigationGestures;
-
+    if (_userAgent) {
+      _webView.customUserAgent = _userAgent;
+    }
 #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 110000 /* __IPHONE_11_0 */
     if ([_webView.scrollView respondsToSelector:@selector(setContentInsetAdjustmentBehavior:)]) {
       _webView.scrollView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;

ios/RNCWKWebViewManager.m

 RCT_EXPORT_VIEW_PROPERTY(automaticallyAdjustContentInsets, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(hideKeyboardAccessoryView, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(allowsBackForwardNavigationGestures, BOOL)
+RCT_EXPORT_VIEW_PROPERTY(userAgent, NSString)
 
 /**
  * Expose methods to enable messaging the webview.

js/WebView.ios.js

         }
         hideKeyboardAccessoryView={this.props.hideKeyboardAccessoryView}
         allowsBackForwardNavigationGestures={this.props.allowsBackForwardNavigationGestures}
+        userAgent={this.props.userAgent}
         onLoadingStart={this._onLoadingStart}
         onLoadingFinish={this._onLoadingFinish}
         onLoadingError={this._onLoadingError}

js/WebViewTypes.js

    * back-forward list navigations.
    */
   allowsBackForwardNavigationGestures?: ?boolean,
+  /**
+   * The custom user agent string.
+   */
+  userAgent?: ?string,
 |}>;
 
 export type AndroidWebViewProps = $ReadOnly<{|