Fix format lookup when JSON or XML are defined as acronyms

[odlp]

When JSON or XML are defined as acronyms the format lookup fails:

ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym "XML"
  inflect.acronym "JSON"
end

ActiveResource::Formats[:xml]
# => NameError (uninitialized constant ActiveResource::Formats::XMLFormat)
# Did you mean?  ActiveResource::Formats::XmlFormat

ActiveResource::Formats[:json]
# => NameError (uninitialized constant ActiveResource::Formats::JSONFormat)
# Did you mean?  ActiveResource::Formats::JsonFormat

The proposed solution uses a hash to lookup built-in formats. For custom formats the implementation falls back to const_get after camelizing the MIME type reference. I didn't find many public examples of custom formats, so not sure these are widely used.

Here's an example of this issue in the wild, from the shopify_api gem:
Shopify/shopify-api-ruby#683

[odlp]

Planning to contribute a PR to Rails shortly to fix this.

Update: rails/rails#42475

[odlp]

Two of the builds are failing because activesupport-7.0.0.alpha requires ruby version >= 2.7.0

[rails-bot]

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[odlp]

I think this is still an issue

[rafaelfranca]

I don't think this is the right fix unfortunately. This removes the possibility of people to register they own format since they would have to modify that BUILT_IN constant. If we want to allow people to change the inflector of those formats and still have active resource working it would be better for us to stop using camelize all together to find the format. Does anyone want to write a new patch?

[seanpdoyle]

I've opened #416 as a re-submission of this proposal.