Add stream version to session creation response

XEP-206 says:
> 4.. Session Creation Response

> The connection manager SHOULD include a 'version' attribute (qualified by the 'urn:xmpp:xbosh' namespace) and a &lt;stream:features/&gt; element (qualified by the 'http://etherx.jabber.org/streams' namespace) in a response as soon  as they are available, either in its session creation response, or (if it has not yet received them from the XMPP server) in any subsequent response.