GetRev makes an immutable copy of doc revision when processing attachments

The revision cache returns mutable revision bodies, this leads to side effects between calls for the same doc revision.

fixes Feature/issue 447
Or even cleaner would be to put the copying into the loadBodyAttachments method itself. You'd need to change its signature so it returns the updated copy of the body, i.e. the return type would be `(Body, error)`.