Schema with union and NativeFromBinary() map

Hi @karrick (I'm hoping you are still around here)

I saw that you've strictly followed the Avro specification in order to develop/maintain this go module. About the JSON encoding, I've seen some complaining about the TextualFromNative() inserting types fields for union Avro types (i.e. #167 and #106). For what I've seen on the specification, these types fields should exist, because TextualFromNative?() use a JSON format.

My question is about NativeFromBinary(). I'm getting type fields in the interface{} returned from it. Shouldn't these types only be added for JSON encoding? The Native is a map (or interface{}) object in go, not a JSON, right?

If an example is required, there is one in the function description.

//             {
//               "type": "record",
//               "name": "LongList",
//               "fields" : [
//                 {"name": "next", "type": ["null", "LongList"], "default": null}
//               ]
//             }
//        (...)
//         binary := []byte{0x2, 0x2, 0x0}
//
//         native, _, err := codec.NativeFromBinary(binary)
//         if err != nil {
//             fmt.Println(err)
//         }
//
//         fmt.Printf("%v", native)
//         // Output: map[next:map[LongList:map[next:map[LongList:map[next:<nil>]]]]]

Start at NativeFromBinary and the LongList union example in the issue description. Compare the returned interface{} structure with the linked Avro JSON encoding specification and related issues #167 and #106. Done means establishing whether union type fields are expected in native binary decoding and recording the project decision.