Fixed bug in modify_assignment command.
Fixed bug in modify_assignment command.

* The due_cutoff parameter was ignored and the due date was being used
  instead.

ucrcsedept/galah,brownhead/galah,galah-group/galah,ucrcsedept/galah,brownhead/galah,galah-group/galah,ucrcsedept/galah,galah-group/galah,brownhead/galah