message: Use difftime_str
diff: @@ -242,13 +242,9 @@ SEXP RApiTypes::ValueToSexp(Value &val) {
  dest_ptr[0] = ((double)val.GetValue<dtime_t>().micros) / 1000;
  NUMERIC_POINTER(res)[0] = val.GetValue<dtime_t>().micros;
  // some dresssup for R
  - RProtector r_time;
  - SEXP cl = r_time.Protect(NEW_STRING(2));
  - SET_STRING_ELT(cl, 0, r_time.Protect(Rf_mkChar("hms")));
  - SET_STRING_ELT(cl, 1, r_time.Protect(Rf_mkChar("difftime")));
  - SET_CLASS(res, cl);
  - // hms difftime is always stored as "seconds"
  - Rf_setAttrib(res, Rf_install("units"), r_time.Protect(Rf_mkString("secs")));
  + SET_CLASS(res, RStrings::get().difftime_str);
  + // we always return difftime as "seconds"
  + Rf_setAttrib(res, RStrings::get().units_sym, RStrings::get().secs_str);
  return res;
  }
  