[BUG] Empty spans/spansets have infinite duration?

Hi,

It's a bit tricky to say which particular module needs to be changed to fix
this bug, hence I am not using RT for reporting it.  The good part is that
all candidate modules appear to have the same author.  :-)

The test case is as follows:

    #! /usr/bin/perl
    use strict;
    use warnings;
    
    use DateTime;
    use DateTime::Duration;
    use DateTime::Span;
    use Data::Dumper;
    
    my $bad_s = DateTime->new(year => 2009, month => 1, day => 12, hour => 1, minute => 0, second => 0);
    my $bad_e = DateTime->new(year => 2009, month => 1, day => 12, hour => 6, minute => 0, second => 0);
    
    my $bad = DateTime::Span->from_datetimes(start => $bad_s, end => $bad_e);
    my $good = $bad->complement;
    
    my $interest_s = DateTime->new(year => 2009, month => 1, day => 12, hour => 4, minute => 30, second => 0);
    my $interest_e = DateTime->new(year => 2009, month => 1, day => 12, hour => 5, minute => 0, second => 0);
    
    my $interest = DateTime::Span->from_datetimes(start => $interest_s, end => $interest_e);
    
    my $r = $interest->intersection($good);
    
    my $d = $r->duration;
    print Dumper $d;
    
    my $d0 = DateTime::Duration->new(seconds => 0);
    print Dumper $d0;

I would certainly expect for the duration $d of the empty set $r to be the
same duration as $d0, that is, zero seconds.  Instead it behaves more like
infinity.

Is that something that you might be interesting in fixing?

Thanks,
\Anton.
-- 
There is no beauty in entropy. -- Eliezer Yudkowsky