Make $responsePrototype protected

Will likely set the response prototype during instantation in
Expressive, and then use that as the default response if none is
provided to `run()`; as such, it cannot be private visibility.

src/MiddlewarePipe.php

@@ -49,7 +49,7 @@ class MiddlewarePipe implements MiddlewareInterface, ServerMiddlewareInterface
     /**
      * @var Response
      */
-    private $responsePrototype;
+    protected $responsePrototype;
 
     /**
      * Constructor