[BUG] http: offsets are relative to the buffer, not to ->som

[BUG] http: offsets are relative to the buffer, not to ->som

Some wrong operations were performed on buffers, assuming the
offsets were relative to the beginning of the request while they
are relative to the beginning of the buffer. In practice this is
not yet an issue since both are the same... until we add support
for keep-alive.