Fixes for Ruby 2.7 keyword arguments warnings

[amatsuda]

This PR updates the code base to be Ruby 3.0 compatible.

After a very long discussion at https://bugs.ruby-lang.org/issues/14183, the Ruby core team finally decided to introduce a slight incompatibility to keyword arguments from Ruby 3.0, i.e. complete separation of keyword arguments literal from Hash literal.

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 (2019-09-14T20:57:39Z master 1edcfd6107) [x86_64-darwin18]
-e:1: warning: The last argument is used as the keyword parameter
-e:1: warning: for `f' defined here

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

$ ruby -ve 'def f(x: nil) p x;end; hash = {x: 1}; f(**hash)'
ruby 2.7.0dev (2019-09-14T20:57:39Z master 1edcfd6107) [x86_64-darwin18]
1

Also, calling super between methods with "last hash argument" and methods with kwargs would warn.

$ ruby -ve 'class A; def f(**opts) p opts; end; end; class B < A; def f(opts = {}) super; end; end; B.new.f'
ruby 2.7.0dev (2019-09-14T20:57:39Z master 1edcfd6107) [x86_64-darwin18]
-e:1: warning: The last argument is used as the keyword parameter
-e:1: warning: for `f' defined here

This PR includes fixes for these Ruby 2.7 warnings (this ensures that this code works with Ruby 3.0) without breaking compatibility with Ruby < 2.7.

[schneems]

Thanks a ton for the PR! Looks like there are some failures on older rubies:

  1) Failure:
273TestManifestUtils#test_warn_on_two [/home/travis/build/rails/sprockets/test/test_manifest_utils.rb:31]:
274--- expected
275+++ actual
276@@ -1 +1 @@
277-"/home/travis/build/rails/sprockets/test/fixtures/manifest_utils/with_two_manifests/.sprockets-manifest-00000000000000000000000000000000.json"
278+"/home/travis/build/rails/sprockets/test/fixtures/manifest_utils/with_two_manifests/.sprockets-manifest-f4bf345974645583d284686ddfb7625e.json"
279
280
281890 runs, 3922 assertions, 1 failures, 0 errors, 4 skips
282
283You have skipped tests. Run with --verbose for details.

[amatsuda]

@schneems The build failures are also happening in the nightly builds
https://travis-ci.org/rails/sprockets/builds,
and #626 has to fix the build!

[schneems]

thanks a ton!