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
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