Fix for allow mutation and corresponding test case

[orfisko]

This is actually my very first pull request to a github repo, I hope it is helpful. Any feedback is appreciated of course! I have run the testcases, they were green.

[Kludex]

Thanks for the PR! :)

• I've continued this on 🐛 Use the negative of allow_mutation on frozen Field #65. I couldn't commit here. :(

When you push a PR on a project, ideally you would create another branch, because the fork has the same configuration as the main branch, so your fork's main branch is protected, and I couldn't push here. 😅

(I've given co-author to you on the other PR)