testcase for simple list parameter separator default/overridden
Implement Request #67101: SolrDisMaxQuery User Fields Methods (uf param)

proto SolrDisMaxQuery SolrDisMaxQuery::addUserField(string field)
proto SolrDisMaxQuery SolrDisMaxQuery::removeUserField(string field)
proto SolrDisMaxQuery SolrDisMaxQuery::setUserFields(string fields)