Generating XML Document from the Avro document, with proper namespace
support.

One last bug with unions; we don't always know the proper reverse
mapping.
Proper XML Schema union translation to Avro union translation when
reading Avro.

This approach works fine for the current mappings, but may show poor
results when date types are added to Avro.  That is because there are 8
different date types in XML, but only one in Avro.