REComposeViewController: use keyWindow instead of delegate.window
REComposeViewController: use keyWindow instead of delegate.window

I'm working on a project, where delegate.window isn't set.
UIApplication.keyWindow should be more generic & work for everybody.

Thanks for cool pod, really useful & convenient!
haiwen/REComposeViewController,takeshineshiro/REComposeViewController,romaonthego/REComposeViewController,kolyvan/REComposeViewController