Fix mail parameter error with old migrations

Summary:
Fixes T9251. Old mail could get saved with bad parameters for two reasons that I can come up with:

• Nothing ever set a parameter on it -- not sure this could ever actually happen; or
• some field contained non-UTF8 data prior to D13939 and we silently failed to encode it.

My guess is that the second case is probably the culprit here.

In any case, recover from this so 20150622.metamta.5.actor-phid-mig.php can proceed.

Test Plan: Same effective patch as user patch in T9251; looked at some mail to make sure it was still pulling parameters properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9251

Differential Revision: https://secure.phabricator.com/D13990