This allows conditionally generating an etag.

It is now tested both via cowboy_req:read_body and
via cowboy_req:cast.

Removes a bad example from the guide of body reading
with period of infinity, which does not work.

While we are identified as a supervisor in the tree,
we no longer manage children processes at that point,
so do not need to trap exit signals. Users can still
enable trap_exit if they prefer to.

We must add it even if we don't end up compressing because
it indicates that we might. This indication doesn't mean
that the user agent's accept-encoding values will ever
result in content encoding being applied.