Make it possible to override true config settings with a false option

This lets you override a true value from `Cloudinary.config` with a false value in the options when calling cloudinary_url, by using `options.fetch(:key, fallback)` instead of `options.delete(:key) || fallback`.

I found this out while debugging an issue with the new attachment flag. It worked for our dev account (where we don't set cname, private_cdn or cdn_subdomain), but not in production (where they are set). It turns out overriding these values doesn't fix my issue with attachments, but I think it's a valid improvement anyway. (A support request has been filed regarding the attachment issue.)

The first commit is for the keys I had issues with. The other commit updates the rest of the options keys for consistency.

I have not bumped the version number. I figure you want to do that yourselves.