Activity emails for cases aren't getting sent to "Assigned To" contacts

1. Create a case for a contact.
2. Go to manage that case, and then create a new "Follow-up" activity.
3. Add a contact to the "Assigned To" field.
4. Save the activity.

At this point you'd assume the contact in the "Assigned To" field gets an email about the activity, but no email is sent.

Note that you have to have "Notify Activity Assignees" turned on for these messages to get sent out.

I've included a patch for it. It's more of a hacky fix than a real fix. I think that probably we should decide on a format for handling lists of ids from forms and then not be doing the array_flip back into the $params array, but then that foreach loop that handles the sending would have to be rewritten because it is also used for sending the emails in the "Send a Copy" section as well.

This bug was introduced in commit 63ace4f7c55ca6de599508423e3ef0f93cd6889b which was a fix for CRM-12512. That fix took out the array_flip on $params['assignee_contact_id'] which some later code depends on.