Fixed issue with a null date represented as today

When no value is set, the value passed is `undefined`. However `moment(undefined)` has the same meaning as `moment()` which returns today's date. What we really want is `moment(null)` for null values. This can be checked by calling `isValid()` on the moment date, a null date should return false.

This prevents the binding handler from setting today's date by default.
Added undefined to method parameters

Prevents pollution of the global undefined object.
Merge pull request #48 from weiran/patch-1

Fixed issue with a null date represented as today