BUG/MINOR: httpclient/lua: don't pop the lua stack when getting headers

BUG/MINOR: httpclient/lua: don't pop the lua stack when getting headers

hlua_httpclient_table_to_hdrs() does a lua_pop(L, 1) at the end of the
function, this is supposed to be done in the caller and it is already be
done in hlua_httpclient_send().

This call has the consequence of poping the next parameter of the
httpclient, ignoring it.

This patch fixes the issue by removing the lua_pop(L, 1).

Must be backported in 2.5.

(cherry picked from commit 01e2be84d746c5203f7c5723bb6ca9e469026c3d)
Signed-off-by: Willy Tarreau <w@1wt.eu>