ui-disable-choice modifies choice objects

[dcopp]

When using a list of objects, the property _uiSelectChoiceDisabled is added to each item.

Plunker: http://plnkr.co/edit/uijTx559uQyRtVuBscj5?p=preview

Click the select control to see the effect in the json dump below it.

Either this side-effect should be documented (if by design ui-select "owns" the list given) or ui-select should not pollute the list elements with its own data.

[pakdev]

This can be an issue when multiple selects want different items disabled from the same list. It appears that ui-select prevents an option from being selected if this property exists.

[wesleycho]

Going to close this as a duplicate of #1200, as that issue seems a little more detailed.