Custom Claims restricts values to Strings

[Levi-Lesches]

Setting custom claims (via Auth.setCustomUserClaims) only allows the user to pass in a Map<String, String>, while the JS equivalent (as well as the official Node.js SDK) allows any value. Isn't it meant to be Map<String, dynamic>? I'm trying to set it to {"isAdmin": true}, but it's not allowed.

[Levi-Lesches]

I could write a PR with the type change, if that would be helpful. There really shouldn't be any errors caused by this, since it is a more relaxed type of the original, and the JS functions are okay with that type anyway.

[Levi-Lesches]

Fixed with #56

[Levi-Lesches]

Just realized we forgot something: Auth.createCustomToken. I don't happen to use it, but it might be helpful to fix that too