onThumbTintColor not updated when thumbTintColor is set to a new value

[ericmarkmartin]

When the thumbTintColor option is changed during runtime, only the off state thumb tint color actually changes because the userDidSpecifyOnThumbTintColor is updated to true during initialization because it is set by the willSet closure of thumbTintColor. I think this is a bug because the onThumbTintColor is not listed in the properties in the README, and likewise nowhere does it document that both properties must be updated in order to make a color change to the thumbView's color.