Fix for discarding a post in non-visual editor.

Fixes: https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/176

/cc @diegoreymendez @sendhil 

I can't place a comment on the relevant line, because it's not new, so I'm going to put it here.  If you agree on my idea to move the code in this method to `didDismissWithButtonIndex`, the case that would probably need to stay here is the one that calls `[actionSheet dismissWithClickedButtonIndex:0 animated:YES];` a few lines above this comment.