return a nil message to get skipped in the pipeline

[jipperinbham]

rather than rely on changing the op type for a message to be skipped, setting it to nil will result in getting skipped here

fixes #260

[coveralls]

Coverage increased (+0.08%) to 59.812% when pulling f9d120b on 260-drop-noops into df11d32 on master.

[trinchan]

This LGTM. However I do see a bug in the skipMsg logic. When skipping a message, we don't update the LastMsg, so if we end up using that for persistence it'll be off by a bit.

[jipperinbham]

good catch, I think until we start using LastMsg that issue may disappear as the state of persisting transporter state is still influx.

[trinchan]

Yeah, it should be okay for now and when we spec out persistence, it'll get caught