Merge pull request #275 from alphagov/refactor-write-api-functions

Refactor write api functions
Capitalize HTTP verbs POST, DELETE
Add tests for PUT to data-set with empty JSON list

- To implement "clear the data-set" we're allowing the user to HTTP PUT
  an empty JSON list, which overwrites the contents of the data set.
- We currently *only* allow an empty JSON list - return an error
  otherwise.
Make behave tests use data group/type URLs

Rather than testing the /data-set-name endpoint (which is deprecated)
we're now testing using the /data/data-group/data-type endpoint.
Fix broken feature test definition
Implement PUT method for empty JSON list

To clear/empty a data-set, you PUT an empty JSON list to the data
endpoint (ie /data/my-data-group/my-data-type).

We currently only allow an empty list - in the future we could extend to
allow setting the entire contents of the data set.

https://www.pivotaltracker.com/story/show/69622004

[Delivers #69622004]