BUG Correct relation saving in DataObject->duplicateRelations()

This caused problems when duplicate() was used in the CMS UI
to duplicate a SiteTree object. Since every object of this type
has a ParentID relation, it copied this empty relation into
new "ghost page".

See https://github.com/silverstripe/silverstripe-cms/issues/689
Merge pull request #2054 from chillu/pulls/dataobject-duplicate-hasone

BUG Correct relation saving in DataObject->duplicateRelations()