Add a default mime extension for .sass files

Fixes https://github.com/rails/rails/issues/16826

Introduced in https://github.com/rails/sass-rails/commit/a62bdf85e56a678aa5fded3d4ed0d10bbc0b7a7b 
Breaks Sprockets because `default_mime_type` is `nil` here: https://github.com/sstephenson/sprockets/blob/b6ce4bf5a8428ae806cad01403c3f94f1ba5b01e/lib/sprockets/asset_attributes.rb#L122

Using `Sprockets::SassTemplate.default_mime_type` at https://github.com/sstephenson/sprockets/blob/b6ce4bf5a8428ae806cad01403c3f94f1ba5b01e/lib/sprockets/sass_template.rb previously worked when monkey patching `Sprockets::SassTemplate` class, but it is `nil` on inherited `SassTemplate < Sprockets::SassTemplate`