HttpStatus: add RFC 6585 status codes

Add newer (Apr 2012) HTTP status codes & their messages to HttpStatus'
getMessage. Adds 428, 429, 431, 511 per RFC 6585, "Additional HTTP
Status Codes".

Change-Id: Ia055880c4b2f3ac43cf5d94dbdd45a3690a58569
Merge "HttpStatus: add RFC 6585 status codes"