Default value for "checked" is undefined, this causes a bug

[ellenaua]

Description

Default value for "checked" attribute is undefined, but documentation says it's "false".
This is important, because when checked=undefined, and invalid=true, this function
https://github.com/PolymerElements/paper-checkbox/blob/master/paper-checkbox.html#L230
is not called and .invalid class is not set on the checkbox.

Expected outcome

Default value for checked attribute should be false
When I set invalid to true, checkbox should get .invalid class.

Actual outcome

Default value for checked attribute is undefined
When I set invalid to true, checkbox does not get .invalid class.

[azariah001]

I've made a hack patch for this problem. If someone could take a look at it and maybe suggest a more efficient way of achieving the same effect that would be brilliant. Holding off on a pull request just yet.

[Igor-Vladyka]

Problem resolved in #172