Support XML and JSON inflector acronyms

Related to #361

Add special-case treatment of JSON and XML acronyms by making direct
references to the `JsonFormat` and `XmlFormat` constants, rather than
relying on inflections.

The original PR included a comment citing that
`ActiveSupport::Inflector::Inflections#clear` was not working as
expected. It was resolved by [a76344f][], which was merged into `main`
prior to the `7.0.0` release. Since the CI matrix currently includes
`7-0-stable` as the minimum version, the code can rely on the resolved
behavior.

[a76344f]: rails/rails@a76344f

Author: seanpdoyle

lib/active_resource/formats.rb

@@ -10,7 +10,11 @@ module Formats
     #   ActiveResource::Formats[:xml]  # => ActiveResource::Formats::XmlFormat
     #   ActiveResource::Formats[:json] # => ActiveResource::Formats::JsonFormat
     def self.[](mime_type_reference)
-      ActiveResource::Formats.const_get(ActiveSupport::Inflector.camelize(mime_type_reference.to_s) + "Format")
+      case mime_type_reference.to_s
+      when "xml" then XmlFormat
+      when "json" then JsonFormat
+      else ActiveResource::Formats.const_get(ActiveSupport::Inflector.camelize(mime_type_reference.to_s) + "Format")
+      end
     end
 
     def self.remove_root(data)

test/cases/formats_test.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class FormatsTest < ActiveSupport::TestCase
+  def test_json_format_uses_camelcase
+    assert_equal ActiveResource::Formats::JsonFormat, ActiveResource::Formats[:json]
+  end
+
+  def test_xml_format_uses_camelcase
+    assert_equal ActiveResource::Formats::XmlFormat, ActiveResource::Formats[:xml]
+  end
+
+  def test_custom_format_uses_camelcase
+    klass = Class.new
+    ActiveResource::Formats.const_set(:MsgpackFormat, klass)
+
+    assert_equal klass, ActiveResource::Formats[:msgpack]
+  ensure
+    ActiveResource::Formats.send(:remove_const, :MsgpackFormat)
+  end
+
+  def test_unknown_format_raises_not_found_error
+    assert_raises NameError, match: "uninitialized constant ActiveResource::Formats::MsgpackFormat" do
+      ActiveResource::Formats[:msgpack]
+    end
+  end
+
+  def test_json_format_uses_acronym_inflections
+    ActiveSupport::Inflector.inflections { |inflect| inflect.acronym "JSON" }
+
+    assert_equal ActiveResource::Formats::JsonFormat, ActiveResource::Formats[:json]
+  ensure
+    ActiveSupport::Inflector.inflections.clear :acronyms
+  end
+
+  def test_xml_format_uses_acronym_inflections
+    ActiveSupport::Inflector.inflections { |inflect| inflect.acronym "XML" }
+
+    assert_equal ActiveResource::Formats::XmlFormat, ActiveResource::Formats[:xml]
+  ensure
+    ActiveSupport::Inflector.inflections.clear :acronyms
+  end
+end