fix(angular): do not copy $$hashKey in copy/extend functions.

Copying the $$hashKey as part of copy/extend operations makes little
sense since hashkey is used primarily as an object id, especially in
the context of the ngRepeat directive. This change maintains the
existing $$hashKey of an object that is being copied into (likewise for
extend).
It is not uncommon to take an item in a collection, copy it,
and then append it to the collection. By copying the $$hashKey, this
leads to duplicate object errors with the current ngRepeat.

Closes #1875