CLEANUP: hlua: use hlua_ref() instead of luaL_ref()

Using hlua_ref() everywhere temporary lua objects are involved.
Those references are expected to be cleared using hlua_unref()
when they are no longer used.
diff --git a/src/hlua.c b/src/hlua.c
index 66018d1..8261099 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -10577,7 +10577,7 @@
 		lua_insert(L, -2);
 		lua_settable(L, -4);
 	}
-	flt_ref = luaL_ref(L, LUA_REGISTRYINDEX);
+	flt_ref = hlua_ref(L);
 
 	/* Remove the original lua filter class from the stack */
 	lua_pop(L, 1);
@@ -10789,7 +10789,7 @@
 
 		/* Save a ref on the filter ctx */
 		lua_pushvalue(s->hlua->T, 1);
-		flt_ctx->ref = luaL_ref(s->hlua->T, LUA_REGISTRYINDEX);
+		flt_ctx->ref = hlua_ref(s->hlua->T);
 		filter->ctx = flt_ctx;
 		break;
 	case HLUA_E_ERRMSG: