The Boss TU-2 (maybe the TU-3 as well) can do this, I'm pretty sure. (I've got the TU-2 - not sure if it's still available tho).
You have a choice of 'bypass' or 'output' - I can't remember which is the correct method for achieving what you want, but one of them does.
I'd check, but I don't have it with me at the mo.