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
Do you always need to make a copy at this point? It looks like the body is only mutated down below by the loadBodyAttachments call, so you could defer the copying till just before that and avoid it in the common case where there are no attachments or attachmentsSince isn't specified.