Set callback after write can cause callback being skipped

When didWriteDataWithTag is called but callback is not yet pushed to
pendingSends dictionary, it will be skipped and cause stream to hang
forever.

ios/TcpSocketClient.m

@@ -180,10 +180,10 @@ - (void)socket:(GCDAsyncSocket *)sock didWriteDataWithTag:(long)msgTag
 - (void) writeData:(NSData *)data
           callback:(RCTResponseSenderBlock)callback
 {
-    [_tcpSocket writeData:data withTimeout:-1 tag:_sendTag];
     if (callback) {
         [self setPendingSend:callback forKey:@(_sendTag)];
     }
+    [_tcpSocket writeData:data withTimeout:-1 tag:_sendTag];
 
     _sendTag++;