Adds test coverage to SpecialistSector and Subcategory

Both of these models were lacking unit testing due to having been
extracted during refactoring. This PR also slightly modifies model
behaviour so as to not hit the content api twice
Improves naming of instance variable used for response memoization
Merge pull request #39 from alphagov/add-test-coverage

Adds test coverage to SpecialistSector and Subcategory