archivematicaCreateMETS2: Map DC term refinements

Archivematica does not support refined Dublin Core, e.g. multi-tiered
terms in the format "dc.description.abstract". If such a term is
encountered, an element is created using the name of the most specific
term. e.g., "dc.description.abstract" is mapped to the element
<dcterms:abstract>.

Refs #6266.
Add JSON conversion test