Don't set cache headers for controllers that only handle POST requests
Call cache method helper 'set_expiry' instead of 'set_cache'

For consistency with other projects.
Stub the Plek response rather than explicitly setting GOVUK_APP_DOMAIN

Doing this has two major benefits:

      - The tests are more portable
      - The specific test is more explicit that it's about providing
        the public URL for the given asset