proto.Marshal() is not a stable form of serialization

[melzhan]

Proto serialization is not stable in client.go. See [the note in the Marshal() documentation] (http://google3/third_party/golang/protobuf/v2/proto/encode.go;l=74;rcl=563382740).

There's no guarantee that the Marshal() result here will match byte-for-byte what was used to produce the signature.
The recommended approach is described at go/protobuf-crypto#authenticating (i.e. store the signed serialized data and verify the signature before deserializing).

[gmacf]

Fixed in #78