Fix tests for Sprockets 3
Fix tests for Sprockets 3

Sprockets 3 supports both `register_transformer` and `register_engine` unfortunately they don't play well with one another i.e. if a "transformer" is registered using `register_engine` it behaves slightly differently than when registered via `register_transformer`. This cause problems because the default engines in `lib/sprockets.rb` that are registered all use `register_engine` so if we want to over-write the existing sass engine we have to do both calls when possible.


sass/sassc-rails,sass/sassc-rails,sass/sassc-rails,bolandrm/sassc-rails,bolandrm/sassc-rails