Fix two problematic bits of code that will be deprecated or broken in Rails 5.

Found in the Rails 5 MR: !5555

These are safe to use in Rails 4.2.7 as well as Rails 5.0.0, so I figured I'd backport them for the sake of making that merge request smaller.

The explanation for the mime_types.rb code is here: https://github.com/rails-api/active_model_serializers/issues/1027#issuecomment-126543577

See merge request !6214