Unmarshaling string to enum produces empty value

[dustin-decker]

Unmarshalling a string to an enum currently unmarshals an empty string because the value is never set and persisted internally.

This MR fixes two issues:

• UnmarshalJSON never updated the internal value after unmarshaling.
• Methods must have a pointer receiver for updates to take effect.

[hhhapz]

Can you please run all of the tests and add all of the changed templates (generated or not) so that the checks can pass? Thanks!

[dustin-decker]

I ran go generate ./... to update the generated schema. All tests seem to be passing already.