Fixed some #iocaine3 bugs that surfaced last night while writing documentation, went to test things again, and found more bugs!

This particular bug is, again, HAProxy-related: if the incoming request has an iocaine-decision header, we mark that as garbage, and route it to the garbage-generating backend directly, skipping the decision making.

However, most of the metrics are handled during decision making, so short-circuiting that also means that metrics won't be set.

I can't do metrics in the short-circuit branch, because that has the potential of running twice.