This will let us branch by abstraction. All existing calls to
`#outstanding_balance` will go through `OrderBalance` hence, will
check the feature toggle.
Note that by default, `OrderBalance` will end up calling
`#old_outstanding_balance`. As the name states, that's exactly what
`#outstanding_balance` was so far. This means no consumers will see any
change in behavior. We just added on item in the call stack (sort of).
This moves them to a more unit-like style where the mailer methods are
the subjects. However, I did so only for the methods that show order
balance and thus we want to be extra sure of their coverage.
DEPRECATION WARNING: `#deliver` is deprecated and will be removed in Rails 5. Use `#deliver_now` to deliver immediately or `#deliver_later` to deliver through Active Job.
The way we set up email sending completely changes with Spree 2. This
change encapsulates that code in a single method so that it can be
changed easily and doesn't create further merge conflicts while we are
still working on the master branch and the Spree upgrade.
