Tidy hits_helper_spec

* Remove duplication around TimePeriod stubbing
* Be explicit about a context that wasn't requiring setup
Make hits period strip use all hits as appropriate

* No longer reverts to summary when a time period is chosen
  on all hits
* further tidy spec to concentrate on conveying intent, not implementation
Merge pull request #473 from alphagov/fix-all-hits-summary-bug

Fix all hits summary bug