Purge cache in save() for both new and existing data-sets

Previously if users tried to access URLs for data-sets that did not exist, Varnish would cache the 404 responses, but when the data-set was later created in Stagecraft the cache would not be purged, and those cached 404 responses would be served up. These cached responses are now purged when the data-set is created.

https://www.pivotaltracker.com/story/show/70022038

[#70022038]
Disabling purging in CSV field serialisation tests