Change default keepalive period to 20 seconds

A 30 second timeout is a commonly used default for websocket
connections (e.g. this is the default in Jetty). Reducing the default
keepalive period from 30 to 20 seconds should more reliably prevent
timeouts in the most common case.

Author: weavejester

src/ring/websocket/keepalive.clj

@@ -16,7 +16,7 @@
    (if (ws/websocket-response? response)
      (let [listener (::ws/listener response)
            executor (:executor options @default-schedule-executor)
-           period   (:period options 30000)
+           period   (:period options 20000)
            task     (promise)]
        (assoc response ::ws/listener
               (reify wsp/Listener
@@ -46,7 +46,7 @@
   "Middleware that adds a periodic ping to WebSocket responses, in order to
   stop them from timing out. Takes the following options:
 
-    :period   - the time in milliseconds between pings (default 30000)
+    :period   - the time in milliseconds between pings (default 20000)
     :executor - a custom ScheduledExecutorService to handle scheduling"
   ([handler]
    (wrap-websocket-keepalive handler {}))