[]byte and []uint8 decoding fails

[klauspost]

Struct:

type ATbyte struct {
    X []byte
}

Fails with this content:

{"X":"VcELoXc4nIW9KPnUoVg="}

Message: "cannot unmarshal tok:string into Go value for offset=27 line=1 char=27"

[pquerna]

encoding/json does do base64 decode of these fields here:
https://github.com/golang/go/blob/master/src/encoding/json/decode.go#L741-L752

(not documented)