Reset IOGate.set_winch_handler when dancing ruby easter-egg terminates

[tompng]

Fix this SystemStackError

irb(main):001> IRB.send(:easter_egg, :dancing)
(CTRL-c)
=> nil
irb(main):002> IRB.send(:easter_egg, :dancing)
(Resize window)
stack level too deep (SystemStackError)
lib/reline/io/ansi.rb:289:in 'block in Reline::ANSI#set_winch_handler'
lib/reline/io/ansi.rb:289:in 'block in Reline::ANSI#set_winch_handler'
lib/reline/io/ansi.rb:289:in 'block in Reline::ANSI#set_winch_handler'
...

IOGate.set_winch_handler needs cleanup: IOGate.deprep(otio) and otio should be a return value of IOGate.prep.
Currently, otio is always nil but may change in the future.

[ima1zumi]

LGTM!