Enhance StreamConnection::isClosed() detection

Sometimes when dealing with amphp StreamConnection::$stream reset to 0

src/Connection/StreamConnection.php

@@ -64,7 +64,7 @@ public function close()
 
     public function isClosed()
     {
-        return null === $this->stream;
+        return !\is_resource($this->stream);
     }
 
     public function send($data)