Bug fix: An attribute's schema type need not be a union if the attribute
is optional.  No longer throws an exception, and only performs the union
to not-union conversion if needed.

The confirmEquivalent function now supports unions, so removing that
case from the if-statement.
Bug fix: if we're using the Avro schema as a selective ARRAY of UNION of
MAP/RECORD filter, every time we skip the children of an element we want
to put the root elements on the stack as valid next children.