Fix DomainObjectReader merges for `@JsonAnySetter` properties

[msparer]

Closes #2407 Related tickets #2407

See #2407 for a more detailed description.

The actual use case, in case anyone is interested is to be able to expose a Map where null values are possible. So we couldn't persist a Map because in our case hibernate doesn't let you use maps with null values (and only keys). So we used an embedded type in a Set and JsonAnyGetter/Setter to do the JSON handling.

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