Do not use keys in DocumentPersister calls to Cursor::toArray()

Allowing iterator_to_array() to use document "_id" fields as array keys is problematic when documents have non-scalar keys (e.g. embedded objects or MongoBinData, which seems to always return "<Mongo Binary Data>" as its string value). See #854 for more context.

This change also reverts ec69eaa2c5fc825a797bcb38e5eb20f4187a8949 from #401. Technically, inverse side relationships should be read-only, so there should be no reason for a set strategy. The previous behavior was also inconsistent with owning side relationships, which used keys from the actual array/object containing the references (not the "_id" fields of the referenced documents).