Add ruby2_keywords to delegating methods

[jochenseeber]

Pull request for #56. Maybe @eregon could have a look at this?

One thing I am not sure about is if calling module_function for a method after calling ruby2_keywords works correctly.

[eregon]

Looks good to me!

[codecov-io]

Codecov Report

Merging #62 (45d2544) into master (5f0e1df) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #62   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           83        89    +6     
=========================================
+ Hits            83        89    +6     

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

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