Only call value method if it has zero arity

[sikachu]

This PR fixes a compatibility problem with Rails when the user specifies parameter with the same name as ActionDispatch::Integration::RequestHelpers methods.

This came up in our application because we are accepting a parameter name options, and when upgrading to Rails 6.1 we starting to get this error:

ArgumentError:
  wrong number of arguments (given 0, expected 1)
# [GEM_HOME]/actionpack-6.1.3.2/lib/action_dispatch/testing/integration.rb:51:in `options'
# [GEM_HOME]/actionpack-6.1.3.2/lib/action_dispatch/testing/integration.rb:429:in `public_send'
# [GEM_HOME]/actionpack-6.1.3.2/lib/action_dispatch/testing/integration.rb:429:in `method_missing'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint/set_param.rb:20:in `value'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint/params.rb:29:in `block in extended'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint/params.rb:27:in `map'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint/params.rb:27:in `extended'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint.rb:107:in `extended_parameters'
# [BUNDLER_GEM_HOME]/rspec_api_documentation-d3892cc73884/lib/rspec_api_documentation/dsl/endpoint.rb:41:in `do_request'

After some investigation, it seems like Rails 6.1 has added options method to ActionDispatch::Integration::RequestHelpers which get included to Request specs:

https://github.com/rails/rails/blob/v6.1.1/actionpack/lib/action_dispatch/testing/integration.rb#L49-L53

      # Performs an OPTIONS request with the given parameters. See ActionDispatch::Integration::Session#process
      # for more details.
      def options(path, **args)
        process(:options, path, **args)
      end

Therefore when rspec_api_documentation tries to call options method to get the value, it accidentally calling this helper method from Rails instead and resulting in ArgumentError.

Since it's a bit tricky to detect where the method was defined, I propose a solution in this PR in which we just check if the value method has an arity of zero (i.e. doesn't take any argument) before calling it. This way, rspec_api_documentation will still automatically call a value method if it's defined or overridden by the user.

[artofhuman]

@davidstosik @jakehow It looks good. Can it will be merged and released?

[ToriK17]

This fix was great! We needed it asap so we had to fork this and add it manually for the time being to regenerate docs quickly. If anyone else is using this in your application remember to add config.response_body_formatter = Proc.new { |response_content_type, response_body| response_body } to your application's configuration. Thanks @sikachu!