Added fix for ignoring shortcut key combinations, added sinon js for unit testing

[nichaladas]

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When the combobox has focus, pressing any shortcut combination made up of alt/ctrl/meta and an alphanumeric character causes the combobox to open the menu and start searching. I've added a fix to ignore the keyup when the control, alt or meta keys are being held.

I also added a unit test which required the inclusion of the latest sinon.js so that I could spy on combobox functions.