FB 33239 add mutable document revision

See https://gist.github.com/mikerhodes/c022e9e44d83c2e4747e for mini spec (objective-c version)
>> @@ -0,0 +1,22 @@
>> +    // NB the key is purely for the user's convenience and doesn't 
>> have to be the same as the attachment name
>
> 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).

Yes, you're right. I agree it'd cause more problems than it'd solve.