Use splitAfter in Websocket.scala

**Issue by [ktoso](https://github.com/ktoso)**
_Monday Jan 04, 2016 at 20:10 GMT_
_Originally opened as https://github.com/akka/akka/issues/19348_

---

As commented there it should simplify the flows a bit:

``` scala
.splitWhen(_.isMessageEnd) // FIXME using splitAfter from #16885 would simplify protocol a lot
```

https://github.com/spray/akka/commit/6980a0ba61291cb71064267c1b7864de96ddd01a#diff-32111086a8e0b3bf2b812bda44d7c0a1R116

Open Websocket.scala and locate the splitWhen(_.isMessageEnd) flow mentioned in the issue. Read the referenced splitAfter change in issue #16885 and the linked commit for context; done means the websocket protocol flow uses splitAfter while preserving its existing behavior.

Written by the indexing model from the issue text.