BUG: http: disable TCP delayed ACKs when forwarding content-length data

BUG: http: disable TCP delayed ACKs when forwarding content-length data

Commits 5c6209 and 072930 were aimed at avoiding undesirable PUSH flags
when forwarding chunked data, but had the undesired effect of causing
data advertised by content-length to be affected by the delayed ACK too.
This can happen when the data to be forwarded are small enough to fit into
a single send() call, otherwise the BF_EXPECT_MORE flag would be removed.

Content-length data don't need the BF_EXPECT_MORE flag since the low-level
forwarder already knows it can safely rely on bf->to_forward to set the
appropriate TCP flags.

Note that the issue is only observed in requests at the moment, though the
later introduction of server-side keep-alive could trigger the issue on the
response path too.

Special thanks to Randy Shults for reporting this issue with a lot of
details helping to reproduce it.

The fix must be backported to 1.4.