Make fixes to the RequestsController spec

The old approach (inherited from when this spec was a test/unit test) interferes with other specs.

Rspec provides a [much cleaner, less framework-invasive way](https://relishapp.com/rspec/rspec-rails/v/3-0/docs/controller-specs/anonymous-controller) of testing anonymous controllers, which doesn't have side-effects.
Fix spec hooks left over from the test/unit port
Merge pull request #179 from alphagov/spec-fixes

Spec fixes