Data type mismatch in angular select2 bindings

There is a bug in the crmUiSelect and crmEntityref Angular directives. When the input is a multiselect, the data pushed into the model by user interaction with the select2 widget is a comma-separated string, whereas pushing a string back into the model programatically will fail because the widget is expecting an array.

I would be happiest with changing multiselects to always i/o an array but this would probably break existing applications, so I've done a more conservative patch.