Pass xmpp.Element object instead of " " on @jabber.send for ping keepalive

When the arg passed to ping is an empty string a TypeError: Object has no method 'root'
is raised within session.js of the node node-xmpp module. Since the basic premise
of the ping method is to simply perform a keepalive, send through a new xmpp.Element
object instead of an empty string. This object has the root() method that the session
send prototype is looking for.
Merge pull request #148 from crashlytics/stanza-root-fix

Pass xmpp.Element object instead of " " on @jabber.send for ping keepalive