DATAREST-328, DATAREST-320 - AssociationOmittingSerializerModifier now correctly considers renamed properties.

Previously, if a persistent property was renamed using e.g. @JsonProperty, the lookup of the PersistentProperty failed as BeanPropertyWriter exposes the final (renamed) property name, not the internal one. We now defensively lookup the correct BeanPropertyDescriptor using the external name and use the corresponding internal one for the PersistentProperty lookup.