DataSet::create: Don't init with DSetCreatePropList::DEFAULT

Because this doesn't create a new PropertyList *based* on the
default, but gives as a handle to the actual default. So afterwards
we would actually be modifying the default list. Doh!
DataSet::create: Actually use the custom creation properties

Earlier we modified the default creation property list, so the code
seemed to work; of course we actually want a new property list
and pass that to the create function.