Avoid wiping non-writable fields in nested objects when modifying a persistent object using PUT

[vierbergenlars]

Because #mergeForPut() fetches the PersistentEntity based on the
class of target, we can't pass Optional<> as source or target, as
that would lead to nested objects (e.g. JPA @Embeddable) are not
considered as persistent entities.

That leads to properties of that are @JsonIgnored or @JsonProperty(access = READ_ONLY) to
be null'ed, because the freshly deserialized source object is used
directly, and those properties are never written to by jackson.

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[mjustin]

I believe this change had the side-effect of causing fields named "id" in directly nested objects in MongoDB documents to no longer update when a PUT request is issued (see #2280 for details).

I noticed this as a new failure that occurred in a project I was upgrading to the newest version of the Spring Framework.