Wrong option removed from select controller

[shahata]

Here's a plunkr that reproduces the issue: http://plnkr.co/edit/CZIUHBrqlz7CS93lbZiH?p=preview

Basically, the problem is that the render() removes the wrong option from the select controller since it assumes that the option that was removed has the same label as the excessive option in existingOptions, but this is only correct if the option was popped from the end of the array.

This was introduced in 2bcd02d

I'm looking into how to fix it, but I'd appreciate any suggestions.
/cc @btford

[btford]

Grrrr.

Maybe we need separate bookkeeping for options added via ngOptions, and to expose a different API publicly versus privately.