Add a baseline test for the middleware extender

Refs #2017.

tests/integration/extenders/MiddlewareTest.php

@@ -26,6 +26,17 @@ class MiddlewareTest extends TestCase
         );
     }
 
+    /**
+     * @test
+     */
+    public function custom_header_is_not_present_by_default()
+    {
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertEquals(200, $response->getStatusCode());
+        $this->assertArrayNotHasKey('X-First-Test-Middleware', $response->getHeaders());
+    }
+
     /**
      * @test
      */