message: dnn: both protobuf readers have similar behavior
  ReadProtoFromTextFile
  ReadProtoFromTextBuffer
diff: @@ -1137,7 +1137,12 @@ bool ReadProtoFromBinaryFile(const char* filename, Message* proto) {
  bool ReadProtoFromTextBuffer(const char* data, size_t len, Message* proto) {
  ArrayInputStream input(data, len);
  - return google::protobuf::TextFormat::Parse(&input, proto);
  +#ifndef OPENCV_DNN_EXTERNAL_PROTOBUF
  + return google::protobuf::TextFormat::Parser(true).Parse(&input, proto);
  +#else
  + return google::protobuf::TextFormat::Parser().Parse(&input, proto);
  +#endif
  +
  }
  