MINOR: http-fetch: Use pointer to HTX DATA block when retrieving HTX body

MINOR: http-fetch: Use pointer to HTX DATA block when retrieving HTX body

In sample fetch functions retrieving the message payload (req.body,
res.body...), instead of copying the payload in a trash buffer, we know
directely return a pointer the payload in the HTX message. To do so, we must
be sure there is only one HTX DATA block. Thanks to previous patches, it is
possible. However, we must take care to perform a defragmentation if
necessary.