fix handling of multiple Content-type headers
This can happen if an error happens while sending
application/mercurial-0.1 content. The error page will
be sent resulting (for at least lighttpd) in the following
headers:
Content-type: application/mercurial-0.1
Content-type: text/html
which result in req.proto = 'application/mercurial-0.1, text/html'
fix issue344
# should print nothing
# should print ? b/x
? b/x
# should print A b/x
A b/x
# should forget b/x
forgetting b/x
# should print nothing