Extend content api test helpers to support options

* Extend `#content_api_has_artefacts_with_a_tag` to take a options hash.
This allows the interface to be flexible enough for a wider range of
testing situations, i.e. artefacts in a specific format.
Replace conditional check for :draft key presence
Use the same style for format key in the details hash
Consistent use of Hash#fetch
Use 'puts' instead of 'p'
Merge pull request #248 from alphagov/add-content-api-artefact-with-format-test-helper

Extend content api test helpers to support options