Rewrite simple test (only grpc output)

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

@@ -592,11 +592,7 @@ func (p *textParser) readStruct(sv reflect.Value, terminator string) error {
 			props = oop.Prop
 			nv := reflect.New(oop.Type.Elem())
 			dst = nv.Elem().Field(0)
-			field := sv.Field(oop.Field)
-			if !field.IsNil() {
-				return p.errorf("field '%s' would overwrite already parsed oneof '%s'", name, sv.Type().Field(oop.Field).Name)
-			}
-			field.Set(nv)
+			sv.Field(oop.Field).Set(nv)
 		}
 		if !dst.IsValid() {
 			return p.errorf("unknown field name %q in %v", name, st)