Removed redundant JSON encode/decode from RCTDataManager

Summary:
@public

For some reason we were manually JSON-encoding the RCTDataManager responses, and then decoding them again on the JS side. Since all data sent over the bridge is JSON-encoded anyway, this is pretty pointless.

Test Plan:
* Test Movies app in OSS, which uses RCTDataManager
* Test any code that uses RKHTTPQueryGenericExecutor to make network requests (e.g. Groups)
* Test the Groups photo upload feature, which uses RKHTTPQueryWithImageUploadExecutor

Examples/Movies/Movies/main.m

@@ -17,6 +17,6 @@
 
 int main(int argc, char * argv[]) {
   @autoreleasepool {
-      return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
+    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
   }
 }