Added newsletter spec

https://www.pivotaltracker.com/story/show/82978346
Wrote rspec tests to cover the remaining parts of the newsletter.rb model file.
The lines:

```ruby
Newsletter.const_set('SEND_AS', :instant)
@newsletter.do_send = true
```

are repeated in multiple expectation blocks. This is a good sign that these lines could be refactored into a `before(:each)` under a meaningful `context` block.