Allow non-boolean return type for "is-getters" with MapperFeature.ALLOW_IS_GETTERS_FOR_NON_BOOLEAN

[Richie94]

On FasterXML/jackson-module-kotlin#575 we discovered, that through filtering of the returntype we got into the situation of completely skipping serializing a field.

Although its common to only have the syntax on writing is only in front of boolean getters, this seems to be a bit too heavy. I would propose, to get rid of this restriction.

[cowtowncoder]

This would change behavior significantly and possibly cause issues for existing users. I'd be happy to have a MapperFeature that controls this behavior (defaulting to old behavior for backwards compatibility), but not for simple change.
I realize this is not necessarily easy to achieve, but seems like something that is needed to avoid unwanted changes in behavior for some users.

EDIT: actually, change might not be super difficult.

[cowtowncoder]

Excellent, thank you again @Richie94! This will go in 2.14.0-rc2 (and 2.14.0 final) and should be very welcome to most users.

I wonder whether Kotlin module should enable this by default?