Ensure compatibility with `ruby --enable-frozen-string-literal`

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/20205

json-schema is used by the rbs gem, which is tested as part of the ruby-core CI, and it's failing with frozen string literals are enabled by default.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.08%. Comparing base (2cc181b) to head (d1d1e48).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #508 +/- ## ======================================= Coverage 90.08% 90.08% ======================================= Files 76 76 Lines 1584 1584 ======================================= Hits 1427 1427 Misses 157 157 ```

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[bastelfreak]

Thanks for the patch @casperisfine

[casperisfine]

Thanks for the extremely timely merge :)

[casperisfine]

As a maintainer I know it's annoying to be asked for a release, but in this case I arrived here because I'm trying to get rbs test suite to pass with --enable-frozen-string-literal and it depend on json-schema. I can use a git gem for now, but not sure they'll be willing to merge my fixes without a proper release cut.

So I'd owe you one if you could cut a release soon 🙏

[bastelfreak]

already on it :) https://github.com/voxpupuli/json-schema/pull/509

[casperisfine]

😻

[mostlyobvious]

considering https://github.com/voxpupuli/json-schema/pull/507 resolved too