title: Lumen route group doesn't work with named route
body: <p>When I define a route inside a group in Lumen framwork it's working well with a direct closure but not with a controller name ; I always get a not found exception.</p>
  
  <pre><code>//Working
  $app-&gt;group(['prefix' =&gt; 'admin'], function () use ($app) {
      $app-&gt;get('users', function ()    {
          //...
      });
  });
  
  //Get 'Class ExampleController does not exist'
  $app-&gt;group(['prefix' =&gt; 'admin'], function () use ($app) {
      $app-&gt;get('users', ['uses' =&gt; 'ExampleController@indexAction']);
  });
  </code></pre>
  
  <p>Thanks in advance.</p>