private function for constructing the query parameter for dfwds
genDfwds(wdcode, valuecode)string value , one of c("zb","sj","reg")
string value , following is the table for available valuecode zb: the valudecode can be gotten by statscnQueryZb() function sj: the valudecode can be "2014" for nd db, "2014C" for jd db. reg: the valudecode is the region code fetched by statscnRegions(dbcode) function
return the queyr string for the http request