Graph.dataDomain() should only consider active series.

If Graph.dataDomain() is not restricted to active series then for
unstacked charts RangeSlider.Preview may set graph.window.{xMin, xMax}
to an incorrect value when series are disabled using the Toggle
behaviour.

This fixes:
https://github.com/shutterstock/rickshaw/issues/497
Test that Graph.dataDomain() only considers active series.

https://github.com/shutterstock/rickshaw/issues/497