Incorrect viewStartDate

This is a fix for a bug, which prevents you to switch to the current month, when the date is set to a day of the previous month. Steps to reproduce the issue:
- create a kalendae with the option: { direction: 'today-past' }
- go back to the previous month, and choose a day which is greater than the current day of month (e.g. select 28 February, when the current day is 4 March)
- close kalendae
- open it again, and the next month button is not available