Simulate app request contenxt when rendering mail previews

Mailers will not expect the request context to be inside debugkit.
Instead we should simulate the app context so that mailer links don't
need `plugin => null` everywhere.

Refs #600

src/Controller/MailPreviewController.php

@@ -20,6 +20,7 @@
 use Cake\Core\Plugin;
 use Cake\Event\Event;
 use Cake\Http\Exception\NotFoundException;
+use Cake\Routing\Router;
 use Cake\Utility\Inflector;
 use DebugKit\Mailer\AbstractResult;
 use DebugKit\Mailer\PreviewResult;
@@ -114,6 +115,11 @@ public function sent($panelId, $number)
      */
     public function email($name, $method)
     {
+        // Clear the plugin attribute from the request instance
+        // Router is holding onto so that we can render mail previews
+        // in a plugin less request context.
+        Router::pushRequest($this->request->withParam('plugin', null));
+
         $plugin = $this->request->getQuery('plugin');
         $email = $this->findPreview($name, $method, $plugin);
         $partType = $this->request->getQuery('part');

tests/TestCase/Controller/MailPreviewControllerTest.php

@@ -44,6 +44,10 @@ public function setUp()
     {
         parent::setUp();
         Plugin::load('DebugkitTestPlugin', ['path' => APP . 'Plugin' . DS . 'DebugkitTestPlugin' . DS]);
+        Router::scope('/', function ($routes) {
+            $routes->connect('/users/:action/*', ['controller' => 'Users']);
+        });
+
         Router::plugin('DebugKit', function (RouteBuilder $routes) {
             $routes->scope(
                 '/mail_preview',
@@ -80,6 +84,7 @@ public function testEmailPartTextContent()
 
         $this->assertResponseOk();
         $this->assertResponseContains('Testing email action.');
+        $this->assertResponseContains('/users/verify/token', 'Should contain URL from app context');
     }
 
     /**

tests/test_app/Template/Email/text/test_email.ctp

@@ -1 +1,2 @@
 Testing email action.
+<?= $this->Url->build(['controller' => 'Users', 'action' => 'verify', 'token']) ?>