Work around mailparse bug (?) with messages that have no terminal newline

Summary: Under some unusual circumstances, mailparse appears to incorrectly discard the last line of some mail messages.

Test Plan:

• Constructed a raw mail with no terminal newline.
• Piped it into mail_receiver.php.
• Saw the last line vanish into the aether.
• Applied patch; repeated; last line survived.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: chad, epriestley

Differential Revision: https://secure.phabricator.com/D12494