MEDIUM: filters/htx: Filter body relatively to the first block

MEDIUM: filters/htx: Filter body relatively to the first block

The filters filtering HTX body, in the callback http_payload, must now loop on
an HTX message starting from the first block position. The offset passed as
parameter is relative to this position and not the head one. It is mandatory
because once filtered, data are now forwarded using the function
channel_htx_fwd_payload(). So the first block position is always updated.