RFC 3261-compliant Warning header

http://tools.ietf.org/html/rfc3261#section-20.43
The Warning header field is used to carry additional information
about the status of a response.  Warning header field values are sent
with responses and contain a three-digit warning code, host name, and
warning text.

The "warn-text" should be in a natural language that is most likely
to be intelligible to the human user receiving the response.

http://tools.ietf.org/html/rfc3261#page-232
Warning        =  "Warning" HCOLON warning-value *(COMMA warning-value)
warning-value  =  warn-code SP warn-agent SP warn-text
warn-code      =  3DIGIT
warn-agent     =  hostport / pseudonym
                  ;  the name or pseudonym of the server adding
                  ;  the Warning header, for use in debugging

http://tools.ietf.org/html/rfc3261#page-182
399 Miscellaneous warning: The warning text can include arbitrary
    information to be presented to a human user or logged.  A
    system receiving this warning MUST NOT take any automated
    action.
Merge pull request #69 from CodeYellowBV/warning-header-rfc3261

RFC 3261-compliant Warning header