Avro's generic api expects GenericEnumSymbol as the runtime type for instances of fields that are of Avro type ENUM. The Avro 1.7 libraries are lax in some cases for handling this, but the 1.8 libraries are strict. We should proactively fix our serialization.
(the lax serialization in 1.7 fails for some nested use in unions, see
AVRO-997 for details)