Datepicker range selector reset creates a visual problem --- FIX.

When clearing values for a datepicker range, it does not clear the previous selection causing a visual UX error due to code in [getClassNames](https://github.com/eternicode/bootstrap-datepicker/blob/master/js/bootstrap-datepicker.js#L578-L585). See [screenshot](https://www.evernote.com/shard/s310/sh/d9dcefca-8441-4837-a6ea-257150432bab/cb05fb7bbb84a42732df4f02976e8657) of issue. 

Adding the committed line of code  `this.range = undefined;` solves this issue.