fixed business day calculation

This will fix the so far broken calculation of business day periods and the corresponding tests reported in #5203.

The expected behaviour found in the present tests is that the date is always rolling forward to the next business day:
```
Friday:   Period('2012-3-09', freq='B') => '2012-03-09'
Saturday: Period('2012-3-10', freq='B') => '2012-03-12'
Sunday:   Period('2012-3-11', freq='B') => '2012-03-12'
Monday:   Period('2012-3-12', freq='B') => '2012-03-12'
```
Can anyone confirm this?