Let fast-avro align with vanilla avro to handle unknown enum value

[gaojieliu]

Right now, Fast-Avro would fail fast during fast-class generation when encountering an unknown enum value, which is different from the vanilla Avro, which would fail the de-serialization when encountering an unknown enum value at de-serialization time. Avro Enum default value is only supported in Avro-1.9+, so having such kind of alignment could help users, who are still using older Avro versions.