Added a string typehint for the $requestTarget argument

Author: Aleksandr Golouz

src/RequestInterface.php

@@ -57,10 +57,10 @@ public function getRequestTarget();
      *
      * @link http://tools.ietf.org/html/rfc7230#section-5.3 (for the various
      *     request-target forms allowed in request messages)
-     * @param mixed $requestTarget
+     * @param string $requestTarget
      * @return static
      */
-    public function withRequestTarget($requestTarget);
+    public function withRequestTarget(string $requestTarget);
 
     /**
      * Retrieves the HTTP method of the request.