Copy tableviews correctly in arraySortedByProperty:

Using _backingView.get_parent().where(&_backingView) leads to a dangling
reference if the new RLMArray outlives the parent, while creating a new query
for the same table view with query.tableview(_backingView) appears to work
fine.
Merge pull request #720 from realm/tg-array-sort-crash

Copy tableviews correctly in arraySortedByProperty: