Fix resource including for STI objects

[stokarenko]

The problem was introduced by #305

When we including the kind of STI resource, we are running in issue with unexpected type attribute:

Given: resource is kind of 'file' type, with 'type' attribute equals to STIDocumentFile
When: we including such resource
Expected: parent_resource.file to be available as usually
Actual: the exception raised

NoMethodError:
       undefined method `[]' for nil:NilClass
     # /gems/json_api_client-1.12.0/lib/json_api_client/included_data.rb:46:in `record_for'

This happens because of

module JsonApiClient
  class IncludedData
    def initialize(result_set, data)
       # ...
       # `parameters_from_resource` will override the actual resource `type` value
       # with value from attribute `type`
       params = klass.parser.parameters_from_resource(datum)
       # ...
       # such overridden `type` is used for grouping.
       # i.e. the resources will be grouped by `STIDocumentFile` instead of `file`
       @data = included_set.group_by(&:type)
       # ...
    end
  end
end

[stokarenko]

@gaorlov FYI )

[stokarenko]

Will fix the missing transform_values method on Monday )

[gaorlov]

Thanks again for your contribution! I only have one comment:
This section of code had grown to be quite complex. I spent about an hour thinking of a good way to simplify it, but couldn't come up with anything meaningful, aside from a couple variable names suggestions which i left in comments.
Again, I super appreciate you taking the time to dive into all the complexities of this piece and contribute.

[stokarenko]

@gaorlov Applied all the things, also added the record to changelog )

Thank you a lot for assistance! )

[gaorlov]

Marvelous. Thanks once again!

[gaorlov]

@stokarenko 1.12.1 is now live.