Clearing a has_many does not nil the object FKs, and loading a has_many adds DB objects in preference to in-memory objects

A response to the unexpected behaviour described in this mailing list thread.

First, the has_many delete_records method is patched to nil the foreign keys of the in-memory objects after they are NULLed in the database.

And second, the association_collection load_target method is patched to use in-memory objects in the collection in preference to objects from the database.