When the underlying IO returns 0 on read, we must stop polling it since it's closed. Otherwise we'll be stuck in a tight loop. this fixes https://github.com/sfackler/rust-openssl/issues/949
h2 test support
This crate includes utilities for writing h2 tests. This is broken up into a
separate crate because it requires the unstable feature flag and to enable
tower-h2 to use the same helpers.