[DatePicker] Selecting a year does not trigger validation

[ghost]

Changing a year enables you to select an invalid date eg a Sunday when weekends are disabled.

Using the Date Picker example on the material-ui web site where selecting weekends is disabled, I selected September, 9, 2016 - a Friday. I then changed the year to 2012 when September 9th of September was a Sunday.

When I click the OK button the Sunday was selected.

I would have expected that pressing OK would have taken be back to the month/day selector and not allowed me to select the Sunday.

[oliviertassinari]

Closing for #4787