Bug 18672: Fix overwriting creation date with modification date when editing list

To reproduce:
- In staff client, create a new list (if you have not already one)
- In db, update virtualshelves and set field created_on to an earlier date,
  something like:
  update virtualshelves set created_on = '2017-04-01 02:09:12' where shelfnumber = 1;
- Go to Home > Lists > Your lists
- Verify that the modified creation date for this list appears in staff client
- Edit list in staff client, save
- Verify that creation date is overwritten by modification date (wrong)

To test:
- Apply patch
- Repeat steps above, verify that creation date no longer is overwritten
  by modification date

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch: Use existing indendation

Koha/Virtualshelf.pm

@@ -65,7 +65,8 @@ sub store {
     $self->allow_change_from_others( 0 )
         unless defined $self->allow_change_from_others;
 
-    $self->created_on( dt_from_string );
+    $self->created_on( dt_from_string )
+        unless defined $self->created_on;
 
     return $self->SUPER::store( $self );
 }