The HTTP 1.1 standard requires an Allow header for 405 Method Not Allowed:
The response MUST include an Allow header containing a list of valid methods
for the requested resource.
So provide such a header when we return a 405 to the user agent.
Signed-off-by: Brian M. Carlson <sandals@crustytoothpaste.net> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>