Add missing keywords for Rails/HttpPositionalArguments

[eitoball]

This PR adds more keywords for Rails/HttpPositionalArgument cop.

The warning for positional arguments exists in both ActionDispatch::IntegrationTest and ActionController::TestCase. There are missing keywords in #3365.

$ git clone [email protected]:rails/rails.git
$ cd rails
$ git checkout v5.0.0.1
$ ag  REQUEST_KWARGS
actionpack/lib/action_controller/test_case.rb
638:      REQUEST_KWARGS = %i(params session flash method body xhr)
642:          args[0].keys.any? { |k| REQUEST_KWARGS.include?(k) }

actionpack/lib/action_dispatch/testing/integration.rb
303:        REQUEST_KWARGS = %i(params headers env xhr as)
305:          args[0].respond_to?(:keys) && args[0].keys.any? { |k| REQUEST_KWARGS.include?(k) }
312:            #{REQUEST_KWARGS.join(', ')}

[bbatsov]

This should also have a changelog entry.