BUG/MINOR: hlua: Skip headers when a receive is performed on an HTTP applet

BUG/MINOR: hlua: Skip headers when a receive is performed on an HTTP applet

When an HTTP applet tries to retrieve data, the request headers are still in
the buffer. But, instead of being silently removed, their size is removed
from the amount of data retrieved. When the request payload is fully
retrieved, it is not an issue. But it is a problem when a length is
specified. The data are shorten from the headers size.

So now, we take care to silently remove headers.

This patch must be backported to all stable versions.