allow private parse method in response middleware

[nic-lan]

Description

Currently the Response Middleware enforces children classes to define public parse method.
This is due to the implicit arg all_included=false passed to respond_to? method

Now I would like to allow also private and protected parse method to allow more flexibility for the children classes.

Todos

List any remaining work that needs to be done, i.e:

• Tests
• Documentation

Additional Notes

When running bundle exec rspec on master i noticed a bounce of failing specs which are not depending on this PR code changes.

The error is:

Failure/Error: @session.request(env[:url].to_s, rack_env)

      NoMethodError:
        undefined method `split' for nil:NilClass

and it refers to lib/faraday/adapter/rack.rb:62

Maybe an issue to fix that should be open.

[iMacTia]

Although I'm not entirely sure as why someone would want to define the parse method as private, I have nothing against it in principle, so happy to merge this in.
I do, however, not entirely like the unnecessary line I commented on below, and would prefer to take it out before merging.

[nic-lan]

change as you required.

Let me know if you have anything else that does not convince you.

[olleolleolle]

All changes implemented, tests pass!

[iMacTia]

Thanks for applying the change! Looks good now 🙌
LGTM