Add an async handler update

app/tests/Handler/AsyncHandlerTest.php

@@ -30,5 +30,10 @@ class AsyncHandlerTest extends TestCase
         $response = $app->handle($request);
 
         $this->assertStringContainsString('<li>1</li>', $response->getBody());
+
+        $request = $this->createRequest('GET', '/test', [ 'Accept' => 'application/json']);
+        $response = $app->handle($request);
+
+        $this->assertEquals('{"list":[1,2,3]}', $response->getBody());
     }
 }