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.

As part of this feature (and because deprecated the `/data-set-name` endpoint),
we changed the feature tests to use the `/data/data-group/data-type` endpoint)

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

[Delivers #69622004]