Respect source XML subfield order during overlay

When adding specific subfields to a field as described by an add
or replace rule we were adding them in rule-order.  Instead, they
should be added in source-XML physical order.  This commit does
that.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>
Stamping 0846: overlay subfield order fix

Respect source XML subfield order during overlay

Signed-off-by: Dan Wells <dbw2@calvin.edu>