Fix delegation on Ruby 2.7

[eregon]

• (*args, **kwargs)-delegation does not work on Ruby 2.7.

See https://eregon.me/blog/2019/11/10/the-delegation-challenge-of-ruby27.html#ruby-3-style-delegation
and https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/#a-compatible-delegation for more details.

Addresses #45 (comment)
and properly fixes #44.

Also fixes jwt/ruby-jwt#396 and in general many SimpleCov usages.

@taichi-ishitani I wonder, was https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/ confusing? It intends to say "use ruby2_keywords; (*args, **kwargs)-delegation does not work on Ruby 2.7". But maybe it needs to be clarified further?

[codecov-io]

Codecov Report

Merging #52 (c1080f7) into master (f065fc3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           82        83    +1     
=========================================
+ Hits            82        83    +1     

Impacted Files	Coverage Δ
lib/docile/fallback_context_proxy.rb	100.00% <100.00%> (ø)

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[taichi-ishitani]

@eregon ,
Thank you for fixing a bug caused by my PR.
I misread the article.

[eregon]

@taichi-ishitani No worry. Do you have any suggestion how to improve the article to be clearer? It seems at least a few people got confused by it.

[ms-ati]

Thank you for this fix @eregon ! I will make a release that includes this

[eregon]

Thanks!

[ms-ati]

@eregon I've just released v1.3.4 to rubygems, which includes this PR. Thanks again.