Implement middleware for faking HTTP methods

Refs #502.

framework/core/src/Api/Middleware/FakeHttpMethods.php

@@ -0,0 +1,32 @@
+<?php
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Api\Middleware;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Zend\Stratigility\MiddlewareInterface;
+
+class FakeHttpMethods implements MiddlewareInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function __invoke(Request $request, Response $response, callable $out = null)
+    {
+        if ($request->getMethod() === 'POST' && $request->hasHeader('x-http-method')) {
+            $fakeMethod = $request->getHeaderLine('x-http-method');
+
+            $request = $request->withMethod(strtoupper($fakeMethod));
+        }
+
+        return $out ? $out($request, $response) : $response;
+    }
+}