I looked through the JIRA tickets but I did not find one that matched
what I just ran into - unions can't have multiple maps, even if each one
has a different value type? Automatic-map-generation is the last feature
I'm adding to AVRO-457 <https://issues.apache.org/jira/browse/AVRO-457>
Avro -> XML conversion) and I was surprised that two maps, each with
different value types, could not co-exist in a union. Is this on purpose?