MINOR: lua: Stop using the lua txn in hlua_http_rep_hdr()

MINOR: lua: Stop using the lua txn in hlua_http_rep_hdr()

In this function, the lua txn was only used to retrieve the stream. But it can
be retieve from the HTTP message, using its channel pointer. So, the lua txn can
be removed from the function argument list.

This patch is mandatory to allow the support of the filters written in lua.