Fixes issue with usage of duck-typed interop middleware

Adds logic to `MarshalMiddlewareTrait::marshalInvokableMiddleware()` to
wrap callable interop middleware in a `CallableInteropMiddlewareWrapper`
instead of a `CallableMiddlewareWrapper`. Doing so highlighted a
problematic test in `MarshalMiddlewareTraitTest` as well, which has now
been corrected.

src/MarshalMiddlewareTrait.php

@@ -148,6 +148,10 @@ private function marshalInvokableMiddleware($middleware, ResponseInterface $resp
             return $instance;
         }
 
+        if ($this->isCallableInteropMiddleware($instance)) {
+            return new CallableInteropMiddlewareWrapper($instance);
+        }
+
         if (! is_callable($instance)) {
             throw new Exception\InvalidMiddlewareException(sprintf(
                 'Middleware of class "%s" is invalid; neither invokable nor %s',

test/Application/MarshalMiddlewareTraitTest.php

@@ -256,7 +256,7 @@ public function testPreparingInvokableInteropMiddlewareReturnsDecoratedInteropMi
 
         $middleware = $this->prepareMiddleware($base);
 
-        $this->assertInstanceOf(CallableMiddlewareWrapper::class, $middleware);
+        $this->assertInstanceOf(CallableInteropMiddlewareWrapper::class, $middleware);
         $this->assertAttributeInstanceOf(TestAsset\CallableInteropMiddleware::class, 'middleware', $middleware);
     }