MINOR: hlua: ambiguous lua_pushvalue with 0 index

In function hlua_applet_http_send_response(), a pushvalue
is performed with index '0'.

But according to lua doc (https://www.lua.org/manual/5.3/manual.html#4.3):
"Note that 0 is never an acceptable index".

Adding a FIXME comment near to the pushvalue operation
so that this can get some chance to be reviewed later.

No backport needed.
1 file changed