Ping interval (#2321)

* fix #1598 fix #2276 - add `pingInterval` to client config

* setPingTimer on ready (instead of on connect)

* use isReady (instead of isOpen) and fix test

* Update client-configuration.md

packages/client/lib/client/index.spec.ts

@@ -862,4 +862,16 @@ describe('Client', () => {
         client.unref();
         client.ref();
     }, GLOBAL.SERVERS.OPEN);
+
+    testUtils.testWithClient('pingInterval', async client => {
+        assert.deepEqual(
+            await once(client, 'ping-interval'),
+            ['PONG']
+        );
+    }, {
+        ...GLOBAL.SERVERS.OPEN,
+        clientOptions: {
+            pingInterval: 1
+        }
+    });
 });