ActorProxy is used for suspending IO tasks

I just realized that celluloid-zmq probably has this same terrible
behavior as celluloid-io had. 

celluloid-io@015fa7dba66e90572645b51df7c22ccc50914020
celluloid-io@c28e5a2a185703401a6494af5b33514868989af0

[Here is one](https://github.com/celluloid/celluloid-zmq/blob/65d23d4779d3ea2b6673cbe547b8fd60e6161f04/lib/celluloid/zmq/sockets.rb#L71), I think.