Geary.RFC822.MailboxAddress: Handle long headers being folded

Remove "\n " sequences from RFC822 text before decoding so that any
long mailboxes/mailbox lists folded before the 80 char limit are
unfolded.

Fixes #1009
3 jobs for mjog/rfc822-header-newlines-3-38 in 4 minutes and 7 seconds (queued for 1 second)