Remove redundant regexp escapes in generators

[resure]

Summary

Remove some redundant regexp escapes in generators, since _ is not a special character and can be used directly.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @eileencodes (or someone else) soon.

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[eileencodes]

ActionCable tests are failing on this branch, is there a difference between Ruby 2.2.4 and 2.3.0 regex? Or maybe it's unrelated, I'm not sure.

[resure]

I was unfortunate enough to fork broken master 💥

Looks like everything is ok now.