RPL bugfix: don't ignore infinite rank DIOs

RPL nodes send infinite rank DIOs to let others know that they don't have a good network connection and expect to get a DIO back with some good network information, but the current code explicitly ignored the infinite rank DIOs. This made new nodes join networks too slowly: they basically needed to wait for a random DIO timer to expire to get information about the network and there was no way for them to signal that they wanted to join the network.