Use PSR-15 middleware standard

This finally adopts the new standardized interfaces instead of the
work-in-progress ones with the `Interop\` prefix.

Since we have now updated to PHP 7.1, we can also use Stratigility
3.0 as the middleware dispatcher.

src/Api/Middleware/FakeHttpMethods.php

@@ -11,15 +11,16 @@
 
 namespace Flarum\Api\Middleware;
 
-use Interop\Http\ServerMiddleware\DelegateInterface;
-use Interop\Http\ServerMiddleware\MiddlewareInterface;
+use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\MiddlewareInterface as Middleware;
+use Psr\Http\Server\RequestHandlerInterface as Handler;
 
-class FakeHttpMethods implements MiddlewareInterface
+class FakeHttpMethods implements Middleware
 {
     const HEADER_NAME = 'x-http-method-override';
 
-    public function process(Request $request, DelegateInterface $delegate)
+    public function process(Request $request, Handler $handler): Response
     {
         if ($request->getMethod() === 'POST' && $request->hasHeader(self::HEADER_NAME)) {
             $fakeMethod = $request->getHeaderLine(self::HEADER_NAME);
@@ -27,6 +28,6 @@ class FakeHttpMethods implements MiddlewareInterface
             $request = $request->withMethod(strtoupper($fakeMethod));
         }
 
-        return $delegate->process($request);
+        return $handler->handle($request);
     }
 }

src/Api/Middleware/HandleErrors.php

@@ -13,12 +13,12 @@ namespace Flarum\Api\Middleware;
 
 use Exception;
 use Flarum\Api\ErrorHandler;
-use Interop\Http\ServerMiddleware\DelegateInterface;
-use Interop\Http\ServerMiddleware\MiddlewareInterface;
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\MiddlewareInterface as Middleware;
+use Psr\Http\Server\RequestHandlerInterface as Handler;
 
-class HandleErrors implements MiddlewareInterface
+class HandleErrors implements Middleware
 {
     /**
      * @var ErrorHandler
@@ -35,15 +35,11 @@ class HandleErrors implements MiddlewareInterface
 
     /**
      * Catch all errors that happen during further middleware execution.
-     *
-     * @param Request $request
-     * @param DelegateInterface $delegate
-     * @return Response
      */
-    public function process(Request $request, DelegateInterface $delegate)
+    public function process(Request $request, Handler $handler): Response
     {
         try {
-            return $delegate->process($request);
+            return $handler->handle($request);
         } catch (Exception $e) {
             return $this->errorHandler->handle($e);
         }