String only keys

In regards to #104 and its ultimate reversion, the decision should be made on whether to make key, string only, or perhaps string | null, in order to provide a non-breaking version.

This will require the following

• [ ] Discussion on the choice
• [ ] Reopening #104
• [ ] Increasing test coverage
• [ ] Releasing a patch version

cc: @StrangeGirlMurph

My take: I prefer the current version. I mean everything else previously would have caused problems and lead to errors as well. Now the errors are just more consistent and there are better guidelines/insurances/whatever that ensure that it breaks at the right spot and people just have to take 4 mins to change the code to align with these changes.