Testing that cache is purged on data-set create

* Adding test_dataset_purges_cache_on_create()
* Resetting mock objects after creating but before saving in test_dataset_purges_cache_on_save()
* Removing test_dataset_does_not_purge_cache_on_create()
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 continue to 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 tests that create data-sets
Merge pull request #123 from alphagov/purge-on-data-set-create

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