Force type safety on .setExpGetter

[esfomeado]

Describe the feature you request

setExpGetter accepts a Object as the value but it should instead accept R since we know the type of the field because of the getter.

This way we would get compilation failures in case of type mismatch instead of failing only at runtime.

(Optional): Suggest A Solution

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If the feature request is approved, would you be willing to submit a PR?

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