Apply wfBCP47() to HTTP response header Content-Language

Apply wfBCP47() to HTTP response header Content-Language

See https://tools.ietf.org/html/rfc7231#section-3.1.3 for Content-Language.
This references https://tools.ietf.org/html/rfc5646 aka BCP 47.

Example:
curl -I https://crh.wikipedia.org/wiki/Ba%C5%9F_Saife | grep -i '^content-language:'
	should contain
Content-language: crh-Latn
	instead of
Content-language: crh-latn

Change-Id: Ice4f40911c3761c2542430935bc1898bc4e7a4d4