refresh: cancel previous call if unfinished

[Oliver-Gibson]

when using the refresh attribute you should be able to cancel the previous call if it is unfinished and a new call is about to happen.

plunkr available here: https://plnkr.co/edit/J5TD85Fff3yARRwnVfsc?p=preview

[Jefiozie]

Hi,

As described in the issue template we would like to have a full description with plunker and version.
In the case we can have a look at the issue and reproduce the issue much quicker.

Please update your issue as requested in the issue template.

The issues forum is NOT for support requests. It is for bugs and feature requests only.
Please read https://github.com/angular-ui/ui-select/blob/master/CONTRIBUTING.md and search
existing issues (both open and closed) prior to opening any new issue and ensure you follow the instructions therein.

Bug description:

Link to minimally-working plunker that reproduces the issue:

Version of Angular, UI-Select, and Bootstrap/Select2/Selectize CSS

Angular:
UI-Select:
Bootstrap/Select2/Selectize CSS (if applicable):

Thanks 😄