Pre-fill of start-end date fails because of the default value store in the session

[jbenito]

If you go here:
​http://indico.cern.ch/roomBooking.py/bookingForm?roomLocation=CERN&roomID=1&day=21&month=2&year=2011
you can see that the start-end date field is pre-filled with the values passed as arguments.
On the other hand, if you fist search for bookings, for a given room during a week. And after you click in a date from the results (not todays date) in order to do a booking on that date, the start-end date is not pre-filled with the corresponding values.