vendor protobuf & grpc

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/github.com/golang/protobuf/proto/text.go

@@ -456,6 +456,8 @@ func (tm *TextMarshaler) writeStruct(w *textWriter, sv reflect.Value) error {
 	return nil
 }
 
+var textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
+
 // writeAny writes an arbitrary field.
 func (tm *TextMarshaler) writeAny(w *textWriter, v reflect.Value, props *Properties) error {
 	v = reflect.Indirect(v)
@@ -519,8 +521,8 @@ func (tm *TextMarshaler) writeAny(w *textWriter, v reflect.Value, props *Propert
 			// mutating this value.
 			v = v.Addr()
 		}
-		if etm, ok := v.Interface().(encoding.TextMarshaler); ok {
-			text, err := etm.MarshalText()
+		if v.Type().Implements(textMarshalerType) {
+			text, err := v.Interface().(encoding.TextMarshaler).MarshalText()
 			if err != nil {
 				return err
 			}