Pipe should not decorate MiddlewarePipe instances

In prepping Expressive to use Stratigility 2, I discovered that
`MiddlewarePipe` instances piped into another `MiddlewarePipe` instance
were being decorated as callable middleware; this should no longer
happen.

This patch adds a test to ensure it does not happen, and updates
`pipe()` to not decorate callable `ServerMiddlewareInterface` instances.

In doing so, the `isInteropMiddleware()` method is no longer relevant,
and I removed it; since it was marked `private`, this is a backwards
compatible change.

src/MiddlewarePipe.php

@@ -128,10 +128,9 @@ public function pipe($path, $middleware = null)
             $path       = '/';
         }
 
-        // Decorate callable middleware as http-interop middleware if we have
-        // a response prototype present.
+        // Decorate callable middleware as http-interop middleware
         if (is_callable($middleware)
-            && ! $this->isInteropMiddleware($middleware)
+            && ! $middleware instanceof ServerMiddlewareInterface
         ) {
             $middleware = $this->decorateCallableMiddleware($middleware);
         }
@@ -266,18 +265,6 @@ private function getCallableMiddlewareDecorator()
         return $this->callableMiddlewareDecorator;
     }
 
-    /**
-     * Is the provided middleware argument http-interop middleware?
-     *
-     * @param mixed $middleware
-     * @return bool
-     */
-    private function isInteropMiddleware($middleware)
-    {
-        return ! is_callable($middleware)
-            && $middleware instanceof ServerMiddlewareInterface;
-    }
-
     /**
      * @param callable $middleware
      * @return \ReflectionFunctionAbstract

test/MiddlewarePipeTest.php

@@ -518,4 +518,19 @@ public function testWillDecorateCallableArrayMiddlewareWithoutErrors()
         $this->assertInstanceOf(CallableMiddlewareWrapper::class, $test);
         $this->assertAttributeSame($middleware, 'middleware', $test);
     }
+
+    public function testPipeShouldNotWrapMiddlewarePipeInstancesAsCallableMiddleware()
+    {
+        $nested = new MiddlewarePipe();
+        $pipeline = new MiddlewarePipe();
+
+        $pipeline->pipe($nested);
+
+        $r = new ReflectionProperty($pipeline, 'pipeline');
+        $r->setAccessible(true);
+        $queue = $r->getValue($pipeline);
+
+        $route = $queue->dequeue();
+        $this->assertSame($nested, $route->handler);
+    }
 }