Keyword arguments have to be explicitly double-splatted in Ruby 2.7+

[kamipo]

The Ruby core team decided to introduce a slight incompatibility to
keyword arguments from Ruby 3.0, i.e. complete separation between
keyword arguments literal and Hash literal.

https://bugs.ruby-lang.org/issues/14183
ruby/ruby#2395

With that, current Ruby master warns when a Hash object was passed in as
keyword arguments:

$ ruby -ve 'def f(x: nil) p x; end; hash = {x: 1}; f(hash)'
ruby 2.7.0dev (2019-09-02T05:20:05Z master 83498854eb) [x86_64-darwin18]
warning: The last argument is used as the keyword parameter
warning: for `f' defined here
1

To eliminate this warning, we need to prefix a "double splat" (**) to
avoid ambiguity:

$ ruby -ve 'def f(x: nil) p x;end; hash = {x: 1}; f(**hash)'
ruby 2.7.0dev (2019-09-02T05:20:05Z master 83498854eb) [x86_64-darwin18]
1

See also:

• Keyword arguments have to be explicitly double-splatted in Ruby 2.7+ ruby-i18n/i18n#486
• https://buildkite.com/rails/rails/builds/63974#7fb9ad05-a745-4022-b634-aa3eb9042b11/6-1865

/usr/local/lib/ruby/gems/2.7.0/gems/mysql2-0.5.2/lib/mysql2/error.rb:55: warning: The last argument is used as the keyword parameter
/usr/local/lib/ruby/gems/2.7.0/gems/mysql2-0.5.2/lib/mysql2/error.rb:94: warning: The last argument is used as the keyword parameter

[sodabrew]

Thanks, I also see this error from the ruby-head build, could you address this as well in the PR?

/home/travis/.rvm/rubies/ruby-head/lib/ruby/site_ruby/2.7.0/rubygems/version.rb:212:
  warning: deprecated Object#=~ is called on Integer; it always returns nil

I suspect the issue is either that the tests should not replace the packaged rubygems version, or should use something newer than 2.7.6?

[kamipo]

Yeah, rubygems 2.7.6 is pretty old version, the fix rubygems/rubygems#2494 is included in newer version rubygems.
I've fixed the warning in this PR as d2a273f and b700550.