ReadMetadataSized took the transport's io.ErrUnexpectedEOF for the end of
a body shorter than its size hint. A document upstream closed before its
Content-Length was served with a 200 and cached, or failed the cooldown
filter with a 502, instead of the cached copy being served. Upstream
bodies are now read through a reader that marks their read errors, and
such a body fails the read as an upstream failure.
