private static void serializeWeightedSetContents(StringBuilder destination, String opName,
                                                     WeightedSetItem weightedSet) {
        serializeWeightedSetContents(destination, opName, weightedSet, "");
    }
Do YQL+ escaping, which is basically the same as for JSON, of the
incoming string to the "quoted" buffer. The buffer returned is the same
as the one given in the "quoted" parameter.

@param in a string to escape
@param escaped the target buffer for escaped data
@return the same buffer as given in the "quoted" parameter
container-search/src/main/java/com/yahoo/search/yql/VespaSerializer.java