Add support for character values

[snicoll]

This was submitted against the original project, see square/javapoet#700. Wondering if there's an appetite to support character values natively?

[pkoenig10]

This seems reasonable to me, we'd be open to a PR if you wanted to contribute.

[snicoll]

Thanks for the feedback. @galderz would you mind submitting your original fix to this repository?

[snicoll]

In case we don't get a reply, I can submit what we do in the Spring Framework.

[galderz]

I've moved on to other things, feel free to pick it up