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
in the docs you stated that trackBy expression and selectAs expression must be the same, otherwise the code won't work as expected, but in this test the two expressions don't match