Don't use ChannelBuffer.array

[`CompositeChannelBuffer.array` throws `UnsupportedOperationException`](http://netty.io/3.5/xref/org/jboss/netty/buffer/CompositeChannelBuffer.html#166). Use `ChannelBuffer.toString(Charset)` instead (cf. [`com.twitter.finagle.http.Message.contentString`](https://github.com/twitter/finagle/blob/master/finagle-http/src/main/scala/com/twitter/finagle/http/Message.scala#L280)).