Suggest opml.xml as name for OPML view by providing the appropriate
header, consistently with similar usage in project_index view.
Signed-off-by: NGiuseppe Bilotta <giuseppe.bilotta@gmail.com>
Acked-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>