FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)
Personally I'm not sure this is worth enforcing. Also if it's a special map subclass then this limits users in their choice of data structure. If they want to replace the entire map then will the have to construct a com.cloudant.AttachmentMap or something?

Similarly on iOS I think this change would break the ability to use a dictionary literal (although I defer to someone with more obj-c knowledge on this one).