Fix kwargs usage for Ruby 2.7

[aubinlrx]

This PR intend to fix Ruby 2.7 deprecation warning:

closure_tree-7.1.0/lib/closure_tree/model.rb:9: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call

I've updated some method names in order to simplify to reflect the new usage.

[foton]

👍
I would like to see this merged.

[foton]

Thanks a lot!