Test for Avro-to-XML-and-back passes!  Storing a local document on disk
with the expected results.

Bug found: if the element is not identifiable via a QName, its
nillability cannot be fetched.
Properly handling nillability.  XmlDatumReader pulls the
XmlSchemaElement references from the state machine generator, and pushes
them to DomBuilderFromSax for reference.