Days on the edge of the year missing the `outside` modifier

[fson]

Days from the next year don't get the outside modifier class, and therefore don't show correctly as grayed out. This only happens at the edge of the year, not at the edges of other months.

Can be reproduced with following code:

        <Calendar firstMonth={12}
                  date={moment('2014-12-01')}
                  weekNumbers={true}
                  size={1} />

[freiksenet]

Duplicate of #4.