Unions with logicalTypes json encoded incorrectly

The spec states "A logical type is always serialized using its underlying Avro type so that values are encoded in exactly the same way as the equivalent Avro type that does not have a logicalType attribute."

But goavro encodes the name of unions as "type.logicalType". The codec will not accept a record where the union is typed as anything else and it outputs this directly to json but that is incorrect. Binary encoding appears correct.

func TestNullableLogicalJSON(t *testing.T) {
		schema := `{
		"type": "record",
		"name": "nester",
		"namespace": "test",
		"fields" : [
			{"name": "a", "type": ["null", {"type": "long", "logicalType":"timestamp-millis"}], "default": null}
		]
	}`

	codec, err := goavro.NewCodec(schema)
	require.NoError(t, err)

	bs, err := codec.TextualFromNative(nil, map[string]interface{}{
		"a": goavro.Union("long.timestamp-millis", time.Date(2006, 1, 2, 15, 04, 05, 565000000, time.UTC)),
	})
	require.NoError(t, err)

	/*
	This is the bug.  This is how the reference implementation encodes the timestamp in a nullable union.
	Which matches the spec that states:
	"A logical type is always serialized using its underlying Avro type so that values are encoded in exactly the same way as the equivalent Avro type that does not have a logicalType attribute."
	
	goavro encodes this as {"a":{"long.timestamp-millis":1136214245565}}
	*/
	require.Equal(t, `{"a":{"long":1136214245565}}`, string(bs))

Start with the inline TestNullableLogicalJSON reproduction and trace codec.TextualFromNative for the nullable logical-type union. Compare its JSON output with the expected {"a":{"long":1136214245565}} and the Avro specification; done means the test passes while binary encoding remains correct.