full support for langb fields

[codenaugh]

Summary

Open RTB 2.6 added support for a new language specification (IETF BCP 47) on BidRequest, Device, and Content objects in the form of an alternate language field called wlangb or langb. A recently merged PR added langb to the Content obj. This PR fully incorporates the new language fields, allowing for complete OpenRTB 2.6 support of all language fields in all associated objects.

BidRequest

wlangb []string

Device

langb string

Content

langb string

Note, aside from simply adding the fields, I've put the 2 lang fields near each other in the respective files. Also moved cattax to be near cat since they are also tightly coupled. Lastly, I made sure the comments were word for word what's in the OpenRTB 2.6 spec, as I felt some important information was left out.

Additional Context from OpenRTB 2.6

Added new language field to support IETF BCP 47
IETF BCP 47 offers additional layers of granularity, for example, differentiating written language
versions of the same spoken language (e.g. Traditional and Simplified Chinese)
https://en.wikipedia.org/wiki/IETF_language_tag

Would like to request a new minor release after merging.