title: Rails error: No HTTP_REFERER was set in the request to this action
body: <p>I am developing a new rails app. but found a error:</p>
  <blockquote>
  <p>ActionController::RedirectBackError in UsersController#show</p>
  <p>No HTTP_REFERER was set in the request to this action, so redirect_to :back could not be called successfully. If this is a test, make sure to specify request.env[&quot;HTTP_REFERER&quot;].</p>
  </blockquote>
  <p>my code:</p>
  <pre><code>rescue_from CanCan::AccessDenied do |exception|
       redirect_to :back
  end
  </code></pre>
  <p>I have some question for this error:</p>
  <ul>
  <li>what is HTTP_REFERER?</li>
  <li>why redirect to back will trigger this error?</li>
  </ul>
  <p>Anyone has good idea?</p>