Improve consistency & naming in MediaController spec
Improve consistency & naming in MediaController spec

* Avoid use of "should" in example names
* Use `have_http_status` matcher vs checking response code
* Check for `:ok` (200 OK) vs `:success` (2XX)
* Improve example names and their consistency
* Split example with two expectations into two examples
* Fixed example name to match expectation

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager