Improve default behavior of reducible! when forced to create a row

Defaults to `map-row-builder`; add examples of `assoc`, 
`datafiable-row`.

src/next/jdbc/result_set.clj

@@ -166,7 +166,7 @@
 
   Supports Seqable which realizes a full row of the data."
   [^ResultSet rs opts]
-  (let [gen (when-let [gen-fn (:gen-fn opts)] (delay (gen-fn rs opts)))]
+  (let [gen (delay ((get :gen-fn opts map-row-builder) rs opts))]
     (reify
 
       clojure.lang.ILookup

test/next/jdbc_test.clj

@@ -141,6 +141,18 @@
   (delete! con :fruit {:id 1})
   (update! con :fruit {:appearance "Brown"} {:name "Banana"})
 
+  (reduce (fn [rs m] (reduced (assoc m :test 42)))
+          nil
+          (reducible! con ["select * from fruit where appearance = ?" "red"]))
+
+  (reduce (fn [rs m] (reduced (rs/datafiable-row (assoc m :test 42) con {})))
+          nil
+          (reducible! con ["select * from fruit where appearance = ?" "red"]))
+
+  (reduce (fn [rs m] (reduced (rs/datafiable-row m con {})))
+          nil
+          (reducible! con ["select * from fruit where appearance = ?" "red"]))
+
   (defrecord Fruit [id name appearance cost grade])
 
   (defn fruit-builder [^java.sql.ResultSet rs opts]