Update for frozen-string-literal friendliness.

[pat]

One minor change that keeps all tests green when frozen string literals are in play in MRI 2.4+.

I've done this testing by adding the pragma comment to all Ruby files in lib and test locally, but have not included those changes in this PR. I'm happy to add them in if you'd like, I just don't want to presume whether that's preferred or not.

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[gsamokovarov]

Hey @pat, thanks for the work. I don't mind the the frozen string literals, as long as they keep the tests green.

[pat]

@gsamokovarov just added a commit with all the pragma comments, plus some changes to line numbers in the integration test (as they were all off by two due to the pragma comment). Tests are still green locally, would expect that to be the case on Travis as well.

The one catch: there's nothing that'll keep those comments in place in the current files, nor ensure they're present for new files. In other projects I find others use Rubocop to enforce this, but that's just something to consider. At the very least, having these comments will ensure that any future invalid uses of frozen string literals will cause test failures in the current files.

[gsamokovarov]

Thanks @pat, I'm merging this as is. The rubocop configuration is a good idea and any PRs in that regard are welcome. We shouldn't diverge from the rails/rails rubocop settings, though.

Anyway, thanks for the contribution! ❤️