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 <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>