/accounts/update_profile_background_image.json is deprecated #341

Which means that Api.UpdateBackgroundImage() is broken. @bear how do you want to handle? Twitter's documentation doesn't say anything about it, but this maybe happened around June or July of last year.

http://mashable.com/2015/07/20/twitter-removes-backgrounds/

I think it was finally killed off sometime after that. Do we want to delete the method entirely or put in a warning? Since Twitter is returning a hard error, I feel like deletion is OK.