Description of problem:


I can use Bug.update() xmlrpc call to add/update a deadline value, but once the value has been set, the Bug.update() can't remove it.


How reproducible:


Steps to Reproduce:
1. Set a valid date in "deadline" field
2. Write a script which uses Bug.update() xmrrpc call to set "deadline" to ""(empty string)

here is an example
my %data;
my %update_data;
$data{"ids"} = [$bug_id];
$update_data{"deadline"} = '';
$data{"updates"} = \%update_data;

$call=$bz_rpc->call('Bug.update', \%data);
    if ($call->fault) {
        my $fault_string = $call->faultstring;
        die "bz xmlrpc call failed: $fault_string ";
    }

  
Actual results:
the value of deadline field is not changed.

Expected results:
the deadline field should be empty


Additional info:
I know perl DBI treats "" as NULL, but I have tried to set $update_date{"deadline"} to other possible empty values ('null', '0000-00-00', undef etc), they won't work either.