Added newsletter spec

https://www.pivotaltracker.com/story/show/82978346
Wrote rspec tests to cover the remaining parts of the newsletter.rb model file.
rather than using `const_set` then having to manually undo them in an `after(:all)` block, you could use RSpec’s `stub_const` method (see https://www.relishapp.com/rspec/rspec-mocks/v/3-1/docs/mutating-constants) which deals with the teardown step automatically.