Changing Error to Warning

Summary:
As discussed with @nicklockwood in the issue #1780, the error should be changed to a warning to not break fetch() to send a POST to a remote API without wanting to parse the reply. E.g. google sends back an empty 1x1px gif when POSTing something to google analytics.
Closes #1860
Github Author: "philipp.krone" <[email protected]>

Author: PhilippKrone

Libraries/Network/RCTNetworking.m

@@ -381,7 +381,7 @@ - (void)sendData:(NSData *)data forRequestToken:(id)requestToken
 
   NSString *responseText = [[NSString alloc] initWithData:data encoding:encoding];
   if (!responseText && data.length) {
-    RCTLogError(@"Received data was invalid.");
+    RCTLogWarn(@"Received data was invalid.");
     return;
   }