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

[MINOR] 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 arose in 1.4 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.

Note that this does not affect 1.3, still this is a bug that's better
fixed than blindly copy-pasted and woken up again.

(cherry picked from commit bbfa7938bd74adbfa435f26503fc10f5938195a3)