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
how about: " should bind to scope value through selectAs expression while tracking object via track by"

The original message is kind of mysterious and I bet that in one week I won't know what it stands for.