Comet transport fix.

NPE were fixed for improve snippet rendering time.

The problem is occurred by ask comet actor for render before it was initialized.
Comet actor installation is
```scala
    // This is how we install our own comet actors in Lift
    S.withAttrs(new UnprefixedAttribute("type", "reactive.web.ReactionsComet", new UnprefixedAttribute("name", page.id, Null))) {
      net.liftweb.builtin.snippet.Comet.render(NodeSeq.Empty)
    }
```
Comet actor creation is calling from 'lift session'
```scala
    LiftRules.cometCreation.append {
      case CometCreationInfo("reactive.web.ReactionsComet", name, defaultXml, attributes, session) if _overrideCometHack.value.isDefined =>
        val comet = _overrideCometHack.value.get
        comet.initCometActor(session, Full("reactive.web.ReactionsComet"), name, defaultXml, attributes)
        comet
    }
```
So our lift comet actor can be initialised in 'any time'. And this moment can be after asking comet actor for render.
```scala
        comet !? (comet.cometRenderTimeout, AskRender) foreach {
          case AnswerRender(_, _, when, _) =>
            page.queue(s"var lift_toWatch = lift_toWatch || {}; lift_toWatch['${comet.uniqueId}'] = $when;")
        }
```
In this case is NPE in CometActor occur.  
Sorry, I didn't know about SIP-18 warning. )