Return 400 Bad Request when request body converter returns incompatible type

[BabalolaBrainiac]

Fixes #1194.
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When text/uri-list is posted to a non-association endpoint, UriListHttpMessageConverter returns a CollectionModel of links. The argument resolver then fails with an internal exception mapped to 500.
This change validates the converter output type and throws HttpMessageNotReadableException (400) instead.

• Added type check in PersistentEntityResourceHandlerMethodArgumentResolver
• Added unit test rejectsRequestBodyIfConverterReturnsIncompatibleType

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