gRPC codec has different semantic on Unmarshal than the default one

[aureliar8]

The README state than an easy way to use the vtprotobuf generated code is to change the default grpc codec to use github.com/planetscale/vtprotobuf/codec/grpc.Codec

However the unmarshal operation of this custom codec has different semantics than the default one: it performs a merge rather than an override.
This is documented in the UnmarshalVT doc but it's easy to miss.

This can be an issue when using grpc streams, because some valid code with the default codec will change behavior:

func (Server) SomeStreamRPC(stream pb.Test_StreamServer) error {
	msg := new(pb.SomeMsg)
	for {
                // msg.ResetVT() <- This is required to keep the same behavior after changing the codec 
		err := stream.RecvMsg(msg)
		if err != nil {
			return err
		}
		// Do something with msg ...
	}
}

I'd suggest that the codec is this repo calls msg.ResetVT() before UnmarhsalVT to keep the semantics of the default codec.
Advanced user can still pass their own codec that does not call ResetVT if they require it.

[vmg]

Good catch. I think this is a sensible change for the default implementation, since it can be overriden by performance-conscious users. Would you mind opening a PR?