`can't modify frozen String: ""` errors when using Ruby 2.7

[LocoDelAssembly]

When using Ruby 2.7 there are can't modify frozen String: "" errors that I was unable to resolve:
https://travis-ci.org/LocoDelAssembly/citeproc-ruby/jobs/629710730#L13932

I've made an attempt to solve this but resulted in moving the problem to cucumber test suite and breaking all ruby versions: https://travis-ci.org/LocoDelAssembly/citeproc-ruby/builds/629716123

I anyone could give me some hints on where the problem could be I may try to resolve it and contribute a pull request.

Thanks

[inukshuk]

I think we can skip this for empty strings for now (the issue is that these default strings are converted from nils, which returns the same frozen empty string on 2.7).

[rbq]

This seems to be fixed in master, but hasn't been released yet?

[inukshuk]

Yes, thanks for the reminder! Pushed to RubyGems just now.

[rbq]

Thanks a lot!