Fix XMLHttpRequest.abort() not functional due to incorrect lazy loadi…

…ng in RCTNetworking ObjC module

Summary:
Introduced in a major lazy loading refactoring: 060664f.

This is especially an issue when aborting a long-lived HTTP connection used as a notification channel, as it will use 1 of the maximum 4 connections per host (default limit defined by iOS' NSURLSession).
Closes #5782

Reviewed By: svcscm

Differential Revision: D2907600

Pulled By: nicklockwood

fb-gh-sync-id: 8406a045904a7ddb61fe1539a0474ec27b5e9e37

Libraries/Network/RCTNetworking.m

@@ -411,7 +411,7 @@ - (void)sendRequest:(NSURLRequest *)request
   task.uploadProgressBlock = uploadProgressBlock;
 
   if (task.requestID) {
-    if (_tasksByRequestID) {
+    if (!_tasksByRequestID) {
       _tasksByRequestID = [NSMutableDictionary new];
     }
     _tasksByRequestID[task.requestID] = task;