fixed issue #56 - invalid property does not update

[ahmedelkilani]

Fixed issue #56 . invalid property is not updated even after calling validate method.

[mcrowder65]

Could we get this merged?

[TomK]

Potentially this is not the correct approach.
IronValidatableBehavior states custom validation should override the method _getValidity and not validate.

Both these solutions fix the issue, but potentially overriding _getValidity is more correct based on documentation.

@arthurevans @e111077 can one of you point this in the right direction?

[e111077]

requesting the summon of the queen of inputs and validation @notwaldorf

[notwaldorf]

It seems in other elements we do both? https://github.com/PolymerElements/gold-cc-cvc-input/blob/master/gold-cc-cvc-input.html#L313