MINOR: hlua_fcn: fix Server.is_draining() return type

Adjusting Server.is_draining() return type from integer to boolean
to comply with the documentation.
diff --git a/src/hlua_fcn.c b/src/hlua_fcn.c
index ed016c0..639a1ef 100644
--- a/src/hlua_fcn.c
+++ b/src/hlua_fcn.c
@@ -1068,7 +1068,7 @@
 		return 1;
 	}
 
-	lua_pushinteger(L, server_is_draining(srv));
+	lua_pushboolean(L, server_is_draining(srv));
 	return 1;
 }