Fixed flash to not wipe out session if session is not set in response

@@ -15,6 +15,7 @@
                      :flash flash)
           response (handler request)
           session  (if-let [flash (response :flash)]
-                     (assoc (response :session) :_flash flash)
-                     session)]
-      (assoc response :session session))))
+                     (assoc (response :session session) :_flash flash))]
+      (if (or session (contains? response :session))
+        (assoc response :session session)
+        response))))

@@ -22,3 +22,15 @@
         response (handler {:session {:_flash message}})]
     (is (nil? (:flash response)))
     (is (nil? (-> response :session :_flash)))))
+
+(deftest flash-doesnt-wipe-session
+  (let [message  {:error "Could not save"}
+        handler  (wrap-flash (constantly {:flash message}))
+        response (handler {:session {:foo "bar"}})]
+    (is (= (:session response) {:foo "bar", :_flash message}))))
+
+(deftest flash-overwrites-nil-session
+  (let [message  {:error "Could not save"}
+        handler  (wrap-flash (constantly {:flash message, :session nil}))
+        response (handler {:session {:foo "bar"}})]
+    (is (= (:session response) {:_flash message}))))