title: Render a controller into a String
body: <p>I can <a href="https://stackoverflow.com/questions/8983245/render-a-template-into-a-string">render a template or a view into a String</a>, but how can I render a controller into a String ? </p>
  
  <p>I would like to perform this kind of operation:</p>
  
  <pre><code>def myAction = {
    ...
    def html = renderToString(controller: 'myController', action: 'myAction', params: [what:'ever'])
    render modify(html)
    ...
  }
  </code></pre>