fix(select): make ngOptions support selectAs and trackBy together

This commit implements a function, "getSelected()", that takes the
selectAs expression into account when determining if a particular
option is selected or not. Previously, the selectAs part of the
ngOptions comprehension expression was being ignored.

Fixes #6564
I would reword this into "working example", and say that now instead of `item.subItem` the `item` is stored in `ngModel`.