Fix an issue with email "Re:" prefix

Summary: See detailed discussion in T1543.

Test Plan:

• Enabled multiplexing.
• Set user A to "enable Re".
• Created a task owned by user B with user A cc'd.
• Verified A got no "Re:" before this patch.
• Applied patch.
• Verified A got "Re:" after this patch.

Reviewers: nh, btrahan, vrana

Reviewed By: nh

CC: aran

Maniphest Tasks: T1543

Differential Revision: https://secure.phabricator.com/D3031