Pivot item active property should support boolean

[ChaseIngebritson]

Currently when a pivot-item within a pivot is provided the active property it provides the active class, regardless of the property's value. Unfortunately, this causes an issue with loops where all items will be set as active since the property disregards whether the value is truthy or falsy.

Here is the line that checks the active property if it helps.

[theGeekyLad]

Hope this @ChaseIngebritson 's PR gets merged soon. At present, I've used a ref on each of the pivot items to conditionally add and remove active on the element. Not very thoughtful.