Fix sending emails for user page edits
Fix #136 (closed)
DocBlock says $replyto can be a string or null: https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_32/includes/user/User.php$4722
Then it passes it unmodified to a function that expects a MailAddress... https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_32/includes/mail/UserMailer.php$106