- some clarifications and fixes to merge(instance, dont_load=True).

- some clarifications and fixes to merge(instance, dont_load=True).
fixed bug where lazy loaders were getting disabled on returned instances.
Also, we currently do not support merging an instance which has uncommitted
changes on it, in the case that dont_load=True is used....this will
now raise an error.  This is due to complexities in merging the
"committed state" of the given instance to correctly correspond to the
newly copied instance.  Since the use case for dont_load=True is
caching, the given instances shouldn't have any uncommitted changes on them
anyway.