fixed retrieving value for group header from booleans

When grouping by a column with boolean values the grouping is not correct.

http://plnkr.co/edit/FeTf4DN7hIx7ZJSdQgTO?p=preview

When grouping by the 'test' column instead of getting 3 groups: 'true', 'false' and 'null' 'false' and 'null' are grouped together into 'null'

https://github.com/angular-ui/ng-grid/blob/master/src/classes/rowFactory.js#L198 should check if val != null