Fix test failures on ruby HEAD

[k-tsj]

ref: https://bugs.ruby-lang.org/issues/21298#note-5

[k-tsj]

NOTE:

Even with this fix, <Struct subclass>.new still doesn't work properly. I'll document it as a "known limitation" later.

[tenderlove]

@k-tsj is there any chance we can release this as a gem? I would like to merge this PR, but Ruby CI is red until this is released. 🙇

[k-tsj]

@tenderlove I updated gems/bundled_gems. Could you try it?

[tenderlove]

@k-tsj that did it! Thank you so much!