fix(ngOptions): correctly watch displayed value for changes

[dremonkey]

ngOptions does not update the displayed value properly if the property being used as the label changes when not using the "select as" or "track by" syntax, i.e <select ng-options="value.label for value in array"></select> will not update if array[0].label is changed but <select ng-options="value.label as value.label for value in array></select> will correctly update.

The bug is demonstrated here: https://jsfiddle.net/76qrsctv/17/

The proposed change fixes this issue.

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[dremonkey]

I signed it!

[googlebot]

CLAs look good, thanks!

[petebacondarwin]

Thanks for highlighting this issue. We needed to provide a unit test too.