http-backend: provide Allow header for 405
http-backend: provide Allow header for 405

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 <738bdd359be778fee9f0fc4e2934ad72f436ceda@crustytoothpaste.net>
Reviewed-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Acked-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git