The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.
Because of this change, we also need to change `background` to `before`
or else these tests will fail.
Why:
User with an empty email value (nil) should not appear in the recipient
list for a given UserSegment at Newsletters or Email Downloads.
How:
Using Enumerable#compact and Enumerable#select to filter out empty emails
Increasing Email Download feature spec and Newsletter model spec to cover
all possible scenarios including the nil email one.
