Merge branch 'bc/http-backend-allow-405'

When the webserver responds with "405 Method Not Allowed", it
should tell the client what methods are allowed with the "Allow"
header.

* bc/http-backend-allow-405:
  http-backend: provide Allow header for 405

http-backend.c

@@ -594,9 +594,11 @@ 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();