Add tests for existing tag request behaviour

Before we start changing these methods, add some
tests for the curated_list and sorted_by methods
on an instance of GdsApi::ContentApi.
Allow tag types in Content API helper's tag methods

This changes the tag, with_tag, curated_list and sorted_by methods in
GdsApi::ContentApi so that they accept an optional extra argument to 
specify the tag_type of the tag.

If no tag_type argument is provided, the existing behaviour is 
maintained.
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.
Merge remote-tracking branch 'origin/content-api-tag-type'