message: Avoiding copy by passing param by reference
  It is best to pass bad_value_ param by reference to avoid copy.
diff: @@ -1938,7 +1938,7 @@ class ParseError
  {
  std::string bad_value;
  public:
  - ParseError(const std::string bad_value_) :bad_value(bad_value_) {}
  + ParseError(const std::string &bad_value_) :bad_value(bad_value_) {}
  std::string toString(const std::string &param) const
  {
  std::ostringstream out;
  