Return correct HTTP status code: 429 Too Many Requests

HTTP status code 403 is usually returned when a resource is permanently forbidden (e.g. file system access, invalid/missing client certificate).
For rate limiting, send status code 429 (see RFC6585: https://tools.ietf.org/html/rfc6585#section-4).
Merge pull request #1 from fhemberger/patch-1

Return correct HTTP status code: 429 Too Many Requests