fetch_attachment does not consider '_rev_'

As far I can see (and test) fetch_attachment does not consider the _rev field when it is passed in the "doc".

This can cause problems: we are going to get always the latest document, avoiding any conflict.

Here my patch:

```
$ diff client.py ~/sources/couchdbkit-0.6.5/couchdbkit/client.py
855d854
< doc = {}
863c862
< resp = self.res(docid).get(name, headers=headers, rev=doc.get('_rev'))

---
> resp = self.res(docid).get(name, headers=headers)
```