Fix/recalculate adjustments chooses best

Added specs to apply promotions in different sequences, asserting that the result is still the same.

Because two of the tests run twice, I added error messages to the specs to indicate which sequence was being run when the spec failed.

Also a few comments and style issues (just made the style consistent with how Spree does private methods), and eliminated the need for a stub in a spec.

The line-item spec still fails (now it fails twice since it is run twice--once for each sequence.)