Support using App as PSR-15 RequestHandler

Implementing the RequestHandlerInterface means the
Slim App is embeddedable to another PSR-15 middleware
stack.

Slim/App.php

@@ -14,6 +14,7 @@
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
 use Slim\Exception\MethodNotAllowedException;
 use Slim\Exception\NotFoundException;
 use Slim\Handlers\Error;
@@ -37,7 +38,7 @@
  * configure, and run a Slim Framework application.
  * The \Slim\App class also accepts Slim Framework middleware.
  */
-class App
+class App implements RequestHandlerInterface
 {
     use MiddlewareAwareTrait;
 
@@ -558,15 +559,30 @@ public function run()
         // create request
         $request = Request::createFromGlobals($_SERVER);
 
+        $response = $this->handle($request);
+
+        $this->respond($response);
+        return $response;
+    }
+
+    /**
+     * Handle a request
+     *
+     * This method traverses the application middleware stack and then returns the
+     * resultant Response object.
+     *
+     * @param ServerRequestInterface $request
+     * @param ResponseInterface $response
+     * @return ResponseInterface
+     */
+    public function handle(ServerRequestInterface $request): ResponseInterface
+    {
         // create response
         $headers = new Headers(['Content-Type' => 'text/html; charset=UTF-8']);
         $response = new Response(200, $headers);
         $response = $response->withProtocolVersion($this->getSetting('httpVersion'));
 
-        $response = $this->process($request, $response);
-
-        $this->respond($response);
-        return $response;
+        return $this->process($request, $response);
     }
 
     /**