MDL-18375 calendar: fixed the issue where changing calendar types and resaving the year in the date/time field caused the year to decrease.

For example, a user may create a date/time profile field and set the 'Start year' field to '2012'.
Another user, using the Hijri calendar, may edit the name of this field. They visit the settings
page and in this case the date '1/1/2012' is converted to '7/2/1433' in Hijri. So, the year '1433'
is then displayed. The user then changes the name of this field and  saves the form. This is where
the issue occurs, as the date '1/1/1433' is converted into Gregorian, which converts to the year
'26/09/2011', so the year '2011' is saved in the DB, not '2012'.