Make `destroy` redirect to agents list

Before this, the `destroy` action was attempting to redirect the user back to the `show` action of the controller, which resulted in an error.

Fixes #531
I'm not so sure about needing to call html_safe now that I've spent some more time with this.

When I run Huginn on my local machine, this is the output when I delete an Agent from a Scenario:

18:48:38 web.1  | Started DELETE "/agents/4?return=%2Fscenarios%2F1" for 127.0.0.1 at 2014-09-27 18:48:38 -0800
18:48:38 web.1  | Processing by AgentsController#destroy as HTML
18:48:38 web.1  |   Parameters: {"authenticity_token"=>"tp1TfXpoFeA1XqOvHErGJ1BNEwHRS+9UsD5kBvBZ8gQ=", "return"=>"/scenarios/1", "id"=>"4"}

You'll see that the DELETE request is html_safe but the resulting params[:return] is back-transformed. If this is the setup, then the origin regex works fine. I can add two specs to the "DELETE destroy" section of the agents spec if needed though.