remove unnecessary type casting

Author: seregazhuk

src/StreamingServer.php

@@ -214,7 +214,7 @@ public function handleRequest(ConnectionInterface $conn, ServerRequestInterface
             $string = $request->getHeaderLine('Content-Length');
 
             $contentLength = (int)$string;
-            if ((string)$contentLength !== (string)$string) {
+            if ((string)$contentLength !== $string) {
                 // Content-Length value is not an integer or not a single integer
                 $this->emit('error', array(new \InvalidArgumentException('The value of `Content-Length` is not valid')));
                 return $this->writeError($conn, 400, $request);