Forms Plugin: Autoresponder config needs to be tweaked

[dotJoel]

It seems to me that when creating a new form, the value for Form.response_email (the field that dictates what form input contains the visitors email address) defaults to "_method".

When editing a form, there is no blank option for this field. There needs to be a blank, default option here.

When this field (Form.response_email) is not empty, the auto-responder function will fire.
Firing this function unexpectedly seems to cause an error, due to not being properly configured, since there is no email address in "_method" input).