HttpLimitReqModule should return 429 not 503

[justink101]

Can we get a flag in HttpLimitReqModule to return a http status code 429 Too
Many Requests instead of 503 Service Temporarily Unavailable since this is a
more accurate status code? I know that 429 is not officially defined, but it
is a defacto-standard.

Perhaps something like:

     limit_req_exceeded 429;

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,248049,248049#msg-248049