Use symbol of mime type instead of object to get correct parser

[meinac]

After registering new :json mime type parsers.fetch can't find the mime type because new mime type is not equal to old one. Using symbol of the mime type as key on parsers hash solves the problem.

Closes #23766.

[rails-bot]

r? @kaspth

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[meinac]

I've fixed the failing tests but registering new parser api has been changed with this way. If you think it's not a good change I can only filter param_parsers in parse_formatted_parameters method to get correct one with it's symbol like

parsers.select{ |mt, p| mt.symbol == content_mime_type.symbol }

@rafaelfranca WDYT?

[rthbound]

Confirmed by modifying process from the associated issue to use @meinac's branch:

mkdir recreate_bug
echo "2.3.0" >> recreate_bug/.ruby-version
cd recreate_bug
echo "source 'https://rubygems.org'" >> Gemfile
echo "gem 'rails', git: 'git@github.com:meinac/rails.git', branch: 'action_pack_json_parser_bug'"  >> Gemfile
gem install bundler --no-ri --no-rdoc
bundle
rails new --api --force .
sed -i "s/gem 'rails.*$/gem 'rails', git: 'git@github.com:meinac\/rails.git', branch: 'action_pack_json_parser_bug'" Gemfile
bundle
spring stop
rails g scaffold Bug
rake db:migrate
echo "Mime::Type.unregister :json" >> config/initializers/mime_types.rb
echo "Mime::Type.register 'application/json', :json, %w( text/x-json application/jsonrequest application/vnd.api+json )" >> config/initializers/mime_types.rb
rails s

And

curl -XPOST -H 'Content-Type: application/vnd.api+json' http://localhost:3000/bugs -d '{"maybe_a_bug": true}'

With these changes, params are again processed correctly.

[rafaelfranca]

Thank you so much for working on this. I merged it in #23816.