codec grpc: fix extra allocations on message unmarshal

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

codec.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"strings"
 
+	b "bytes"
+
 	"github.com/golang/protobuf/jsonpb"
 	"github.com/golang/protobuf/proto"
 	"github.com/micro/go-micro/codec"
@@ -81,7 +83,7 @@ func (jsonCodec) Marshal(v interface{}) ([]byte, error) {
 
 func (jsonCodec) Unmarshal(data []byte, v interface{}) error {
 	if pb, ok := v.(proto.Message); ok {
-		return jsonpb.UnmarshalString(string(data), pb)
+		return jsonpb.Unmarshal(b.NewReader(data), pb)
 	}
 
 	return json.Unmarshal(data, v)