Fixes an `undefined method `gsub' for nil:NilClass` exception when fetching source
data for this url:
* /uploads/new?url=https%3A%2F%2Fuiti-ukumo.tumblr.com%2Fpost%2F156157443158%2F2016-12
Bug introduced in 3c524aa5: `url` wasn't initialized due to `@url = url`
being removed from Sources::Site#initialize.