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 <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>

http-backend.c

@@ -564,9 +564,11 @@ int main(int argc, char **argv)
 
 			if (strcmp(method, c->method)) {
 				const char *proto = getenv("SERVER_PROTOCOL");
-				if (proto && !strcmp(proto, "HTTP/1.1"))
+				if (proto && !strcmp(proto, "HTTP/1.1")) {
 					http_status(405, "Method Not Allowed");
-				else
+					hdr_str("Allow", !strcmp(c->method, "GET") ?
+						"GET, HEAD" : c->method);
+				} else
 					http_status(400, "Bad Request");
 				hdr_nocache();
 				end_headers();