I ran across this while working on the spec.

https://github.com/http2/http2-spec/issues/281

Setting the max header size to 0 does not clear out all encoder state.

In particular, elements to the reference set that are pointing to static
table elements are not cleared out.

There are a couple of obvious ways of fixing this:
1) Include the space used in the reference set in the overhead (e.g.
2-bytes per reference)
2) explicit SETTING for clearing the compression state


I prefer #1.