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.  
Right. It was hard to understand at the first time. So source code must be without symbolic names to be more readable, especially, in case of failure documentation. But I've seen it very useful for me after some time.