It seems this is actually allowed by the RFCs; although it is intended
as HTTP/1.0 backward compatibility, and HTTP/1.1 servers (backends)
really should do better.
Even if they shouldn't (due to HTTP/1.0 or Connection; close) some
backends send HTTP/1.1 without Connection: close, and use Content-Length
to signal end of response (and don't close the connection, as they wait
for another request).
Now Content-Length is used to find the end of the response (chunked
transfer-encoding was already supported).
mod_proxy now signals HTTP/1.1, but also sends "Connection: close": it
doesn't reuse the connection yet.