HPACK wasn't emitting header table entries to spec.

More expensive, but more importantly more correct implementation  of HPACK.  Previously, we were not immediately emitting headers that were toggled back on in a later header block.  This caused a problem because during eviction, where we would lose the header.

We didn't catch this earlier as we wrote our hpack tests to the spec examples, which didn't cover this scenario.  Also, our tests didn't do a lot of stateful checks.

During #640, @jpinner explained where in the spec this scenario is described and how to fix it.  Many thanks for the help!