AJAX triggered drupal_goto() in overlay causes double redirect

Whenever you start a batch progress bar inside overlay, your redirect in the end (drupal_goto()) is fired twice: first for a redirect to overlay, then for a redirect away from overlay and to the regular page.

Steps to get a testing environment:

1. Download ManyMail 7.x-1.0-beta6
2. Get PHPMailer as described in the INSTALL.txt
3. Get and enable Libraries API 7.x-2.x
4. Enable just ManyMail
5. Have Overlay enabled (duh)
6. Fill out bogus SMTP settings at /admin/config/manymail/config/smtp (or don't if it already says your settings don't validate)
7. Go to /manymail/all and send an e-mail

Now, the e-mail you attempted to send will fail early because your SMTP settings were invalid.
So after about a minute, ManyMail will notice your mail was interrupted and will allow you to resend it.

This is where the bug manifests itself: starting a batch inside an overlay will end in a double redirect (perhaps it manifests itself with any drupal_goto() inside an overlay?).

Steps to reproduce

• If you go to /admin/config/manymail/logs and resume the mail, all works fine (well the mail still fails of course).
• If you go to /any-page#overlay=admin/config/manymail/logs and resume the mail, you'll get the 'Failed to send' error for a millisecond within the overlay, before it redirects to the same page outside of the overlay (and thus without the error). Pressing the Back button will bring you back to the overlay page with the error.