Allow other namespaces under metadata element

processContents is being left as "lax" to avoid requiring people to
write other schemas for validating with custom namespace elements.
Requiring "strict" validation of other namespaces ends up requiring a
lattice structure of schema files importing one another.

Reported-by: Cliona Curley <cliona.curley@ucd.ie>
Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>