Boxing Day is not being shifted when it falls on a Saturday

In the UK and Northern Ireland, holiday days are observed in lieu of Christmas Day and Boxing Day if either of those days falls on a weekend. The current logic for shifting these dates (in `workalendar.europe`) is only triggered when Christmas Day falls on a weekend and so does not account for the scenario where Christmas Day falls on a Friday, meaning that Boxing Day is on a Saturday and should be shifted to the following Monday.

Please review my changes in `workalendar.europe`, where I've added an additional clause to account for this particular scenario. I've also added a unit test in `test_europe.UnitedKingdom`.

Hope this is correct.

P.S. thanks for the great library!

- D