Re-using a Mailing that was sent to a group which is no longer a Mailing List breaks Recipients field

When a sent mailing that was sent to a group that is no longer a mailing list is re-used, the group won't show up in the recipients field when creating the mailing. That group cannot be removed. However, Civi will still send the email to all the contacts in that group if you send the Mailing, whether or not you add additional Groups to the recipients field.

To reproduce:

1. I set up a Mailing List group called "Test Group 1" on the demo site here: http://d46.demo.civicrm.org/civicrm/group/search?force=1&context=smog&gid=8
2. I created and sent a mailing to this group here: http://d46.demo.civicrm.org/civicrm/mailing/report?mid=17&reset=1 screenshot is called TestEmail_1.png
3. Edited this group's settings and unchecked the "Mailing List" checkbox on the "Test Group 1" group.
4. Went to Scheduled and Sent Mailings, and clicked the Re-use link to the right of the mailing I'd sent earlier.
5. There are no recipients displayed in the Recipients field, yet the recipient count to the right of that field shows that ~1 contact will receive the mailing.
6. Tests on our own server confirm that sending the mailing without adding any additional recipients will send to the indicated number of contacts.
7. See attached screenshot TestEmail_2.png which was captured after doing nothing except clicking the re-use link as outlined in step 4.

I realize that this wouldn't be common practice for an organization (to stop using a particular group as a Mailing List) so the issue is probably trivial and/or limited in impact. Some of our chapters (who only have access to certain groups via ACLs) have been able to send CiviMail to non-mailing list groups in previous versions of Civi (a bug that we never tracked down) and now that CiviMail has an revamped UI we're starting to see the described behavior since chapter folks are still re-using their older mailings. There is a separate issue concerning from email address that seems to be caused by the above phenomenon that I'll detail in another issue ticket. See CRM-17321

IMBA, finding new and creative ways to break CiviCRM since 2010.