message: Fix toString->toUtf8String
diff: @@ -1037,7 +1037,7 @@ generateVariableEqualsConstant(const Dataset & dataset,
  (dataset, columnName, filter,
  "generate rows where var '" + variable.columnName.toUtf8String()
  + "' matches value '"
  - + constantValue.toString() + "'");
  + + constantValue.toUtf8String() + "'");
  }
  static GenerateRowsWhereFunction
  