Case class generation issue with union of only one type

[joelvim]

The default AvroScalaTypes union value is OptionEitherShapelessCoproduct, and when the schema has a union with only one type (["long"] for instance) the code generated is Long (in standard mode) and the union notion is lost. It creates an inconsistency with avro4s schema inference.

An example:
Avro schema

{
  "type":"record",
  "name":"TestData",
  "namespace":"my.namespace",
  "fields":[
    {"name":"longValue","type":"long"},
    {"name":"unionValue","type":["long"]},
    {"name":"stringValue","type":"string"}
  ]
}

Generated case class (the union type is lost)

final case class TestData(longValue: Long, unionValue: Long, stringValue: String)

Expected generated code (at least one possibility)

final case class TestData(longValue: Long, unionValue: Long :+: CNil, stringValue: String)