Correct e_mail_formatter_canon_header_name()

The function is supposed to make ASCII header names canonized
in a form that the first letter it capital and then also each after
the dash letter, but there was a bug when the header name was already
in this canonized form, then the capital letter after the dash
had been made lower case.

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1557329