Pubsub discovery seem to be broken when using MDNS only

#### Version information:

`github.com/ipfs/go-ipfs v0.7.0`
#### Description:

When subscribing and publishing to the same topic, 2 IPFS nodes connected to each other do not receive messages. When I use the swarm api to list connected peers, the other peer does appear in the list however when listing pubsub peers the list is empty.
When I swap with my own libp2p host, it works as expected. You can checkout [this repo](https://github.com/tchardin/ipfs-pubsub-test) which features a basic example demonstrating the issue. Simply run the app in separate terminal windows and you can see the swarm is connected but not pubsub. You can also uncomment the [libp2p host](https://github.com/tchardin/ipfs-pubsub-test/blob/0ab3111b5d38d21755ec4eee0a4d83daecc62c41/main.go#L306) to see how it should work.
Please let me know if I am misunderstanding or missing a configuration to enable pubsub discovery on top of swarm.
Thanks for your time!