fix(select): Fix to work with async changes to menu items

As mentioned in https://github.com/mgcrea/angular-strap/pull/85 the field is not being updated when model used by ng-options is updated.

I am using this directive with a ng-options content fetched by $http and the bootstrap-select field never refreshes.

I have no idea why the test regarding this is passing with the existing implementation. From reading the code it looks like it wouldn't and from experimenting with console log I can see that the refresh is never called on the object after the model is updated. Consequently I couldn't find a way to capture this bug for a new test.