Replace ControllerInterface with PSR-15 interface

The custom interface already had the same signature as the
one from the standard (except for the return type hint), so
why not use that one now? :)

src/Http/Controller/AbstractHtmlController.php

@@ -12,16 +12,18 @@
 namespace Flarum\Http\Controller;
 
 use Illuminate\Contracts\Support\Renderable;
+use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\RequestHandlerInterface;
 use Zend\Diactoros\Response\HtmlResponse;
 
-abstract class AbstractHtmlController implements ControllerInterface
+abstract class AbstractHtmlController implements RequestHandlerInterface
 {
     /**
      * @param Request $request
      * @return HtmlResponse
      */
-    public function handle(Request $request)
+    public function handle(Request $request): ResponseInterface
     {
         $view = $this->render($request);
 

src/Http/Controller/ControllerInterface.php

@@ -1,23 +0,0 @@
-<?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\Http\Controller;
-
-use Psr\Http\Message\ServerRequestInterface;
-
-interface ControllerInterface
-{
-    /**
-     * @param ServerRequestInterface $request
-     * @return \Psr\Http\Message\ResponseInterface
-     */
-    public function handle(ServerRequestInterface $request);
-}

src/Http/ControllerRouteHandler.php

@@ -11,11 +11,11 @@
 
 namespace Flarum\Http;
 
-use Flarum\Http\Controller\ControllerInterface;
 use Illuminate\Contracts\Container\Container;
 use InvalidArgumentException;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\RequestHandlerInterface;
 
 class ControllerRouteHandler
 {
@@ -55,15 +55,15 @@ class ControllerRouteHandler
 
     /**
      * @param string $class
-     * @return ControllerInterface
+     * @return RequestHandlerInterface
      */
     protected function resolveController($class)
     {
         $controller = $this->container->make($class);
 
-        if (! ($controller instanceof ControllerInterface)) {
+        if (! ($controller instanceof RequestHandlerInterface)) {
             throw new InvalidArgumentException(
-                'Controller must be an instance of '.ControllerInterface::class
+                'Controller must be an instance of '.RequestHandlerInterface::class
             );
         }