[BUG] buffer_replace2 must never change the ->w entry

[BUG] buffer_replace2 must never change the ->w entry

This function is used to move data which is located between ->w and ->r,
so it must not touch ->w, otherwise it will displace pending data which
is before the one we're actually overwriting. The issue arises with
some pipelined responses which cause some part of the previous one to
be chopped off when removing the connection: close header, thus
corrupting last response and shifting next one. Those are detected
in the logs because the next response will be a 502 with flags PH.