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/Http/Middleware/SetLocale.php

@@ -12,11 +12,12 @@
 namespace Flarum\Http\Middleware;
 
 use Flarum\Locale\LocaleManager;
-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 SetLocale implements MiddlewareInterface
+class SetLocale implements Middleware
 {
     /**
      * @var LocaleManager
@@ -31,7 +32,7 @@ class SetLocale implements MiddlewareInterface
         $this->locales = $locales;
     }
 
-    public function process(Request $request, DelegateInterface $delegate)
+    public function process(Request $request, Handler $handler): Response
     {
         $actor = $request->getAttribute('actor');
 
@@ -45,6 +46,6 @@ class SetLocale implements MiddlewareInterface
             $this->locales->setLocale($locale);
         }
 
-        return $delegate->process($request);
+        return $handler->handle($request);
     }
 }