Fix customize_gemfiles on Ruby 3

[yevhenii-ponomarenko]

Summary

#191 added an ability to customize headers in gemfiles generated by Appraisal, as well as creating the strings using single quotes. But there's a small bug with the implementation that prevents it from working on Ruby 3:

def customize_gemfiles(&_block)
  Customize.new(yield)
end

With changes in Ruby 3, the Hash yield'ed from the block needs to be converted into keyword arguments with **.
This PR resolves the issue and provides tests to validate this behaviour.

This addresses #213.

[joe-sharp]

@nickcharlton you free to merge this?

[nickcharlton]

Great! Thanks for contributing this, and sorry for the delay.

@joe-sharp, thanks for the ping, I appreciate it.

[luke-hill]

@nickcharlton - This looks more like the issue I found

[nickcharlton]

Thanks! I'm keen to do a release with this, but I don't want to do so until I've gotten to the bottom of Bundler incompatibilities. In the short-term, you might wish to pin against the commit where this has been merged in.

[tisba]

@nickcharlton Do you suspect more issues? Or are we basically talking about #218?

[nickcharlton]

We're talking about #218. There are a few deprecation warnings I've been looking into, but I don't think those will be a problem just yet.

[tisba]

I would suggest to switch to absolute paths for the tests when referencing local git repositories. I'm still not 100% sure I understand Bundler intentions (see comments on rubygems/rubygems#4475), but I'll try to spend some more time on it to understand this better.

If for appraisal this is just about its own specs, I'd just change them, tbh.

[nickcharlton]

Yeah, just changing them seems like a good idea. It's always going to be awkward working through private APIs like we seem to be.