argument types of def foo(...)

[shugo]

Currently, argument types of def foo(...) is (*untyped), but it will be changed to (*untyped, **untyped **) if ruby/ruby#6818 is merged.

The current behavior depends on the implementation details of ..., which uses ruby2_keywords internally, and I believe (*untyped, **untyped) is right.

@soutaro What do you think?