If source.method is "post", use HTTP POST method (facebook#21010)

Summary:
The React Native Web View takes a method option to determine if HTTP GET or HTTP POST should be used.
For iPhone this is not case sensitive, meaning that `method: "post"` is allowed.
For Android "post" is not understood and therefore the request will be using HTTP GET.

I suggest we ignore case for the method, or throw an Exception.
Pull Request resolved: facebook#21010

Differential Revision: D9700895

Pulled By: hramos

fbshipit-source-id: 3fa904e7946dd40958cb44f9a234ee66cb68fab2

ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java

@@ -525,7 +525,7 @@ public void setSource(WebView view, @Nullable ReadableMap source) {
         }
         if (source.hasKey("method")) {
           String method = source.getString("method");
-          if (method.equals(HTTP_METHOD_POST)) {
+          if (method.equalsIgnoreCase(HTTP_METHOD_POST)) {
             byte[] postData = null;
             if (source.hasKey("body")) {
               String body = source.getString("body");