Fix >2gb streamed HTTP responses

src/WebSocketContext.h

@@ -316,7 +316,7 @@ private:
 
             /* We store old backpressure since it is unclear whether write drained anything,
              * however, in case of coming here with 0 backpressure we still need to emit drain event */
-            int backpressure = asyncSocket->getBufferedAmount();
+            unsigned int backpressure = asyncSocket->getBufferedAmount();
 
             /* Drain as much as possible */
             asyncSocket->write(nullptr, 0);