Ensure backends loads correct mappings

The `backend_name` variable is a symbol, not a string. So it was not
finding the mappings when looking them up. Unfortunately there's a
default fallback so this was silently failing.

Fixed the implementation and added a unit test for the Backends class.
Consistent quote style
Merge pull request #52 from alphagov/bugfix-use-correct-mappings

Ensure backends loads correct mappings