7.0 fix timesheet fulfill crash

fix a crash with the timesheet fulfill wizard which was passing datetime objects instead of strings 

add a test showing the bug is fixed. 
OK, don't worry, I try to explain:

datetime_current = "2014-01-01"
date_end = "22:46:00"

datetime_current + date_end = "2014-01-0122:46:00", not "2014-01-01 22:46:00"

Regards.