(use '[clojure.contrib.str-utils])

(defn map->querystring [m]
  (let [join-pair (fn [[k v]]
                    (str-join "=" [(.toUpperCase (name k)) v]))]
    (str-join "&"
              (reduce (fn [acc pair]
                        (conj acc (join-pair pair))) [] m))))

(map->querystring {:class "RES" :mlsnum "23823" :format "COMPACT"})

; => "CLASS=RES&MLSNUM=23823&FORMAT=COMPACT"