call initialize in super to supress warnings for @dead.
call initialize in super to supress warnings for @dead.

The following warning messages are printed continuously:
lib/celluloid/io/mailbox.rb:25: warning: instance variable @dead not initialized

dilumn/celluloid-io,marshall-lee/celluloid-io,celluloid/celluloid-io,kenichi/celluloid-io,ioquatix/celluloid-io