Fix 2.7 deprecation warning in middleware/stack

[Legogris]

Attempt at fixing #2122

See https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/ (Handling argument delegation)

[grape-bot]

	1 Warning
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

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[dblock]

Thanks, I think this was the last one.

[Legogris]

Thanks, I think this was the last one.

Thanks for merging! Almost ;)
#2121

[eregon]

This is unfortunately not correct, *args, **opts-delegation is not correct in some cases on 2.7.
The only correct way to do delegation on Ruby 2.7 is using ruby2_keywords + *args-delegation.
This is also what the Rails ActionDispatch::MiddlewareStack does (https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/middleware/stack.rb, which is very similar).

See https://eregon.me/blog/2019/11/10/the-delegation-challenge-of-ruby27.html for why it's broken on 2.7,
and as you mention above https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

A compatible delegation that works on Ruby 2.6, 2.7 and Ruby 3
In short: use Module#ruby2_keywords again.

FWIW, I'm coming from oracle/truffleruby#2159

[dblock]

Interesting. Can we get rid of all of this complexity?

[eregon]

Yes, if you use ruby2_keywords you only need a few ruby2_keywords calls on methods accepting *args.

[dblock]

@eregon care to PR a fix?

[eregon]

#2132

[Legogris]

@eregon Derp, great that you caught this!