Refactor tag type key lookup in Content API helper
Refactor tag type key lookup in Content API helper

This doesn't reduce the amount of code, but it reduces the duplication of
the ternary statement and use of the CGI.escape method on the tag_type
by moving it to a private method.
alphagov/gds-api-adapters,bitzesty/gds-api-adapters,bitzesty/gds-api-adapters