Keyerror when saving security docs

This issue is related to #134.

There still seems to be a problem when saving security documents. I get

```
File "/usr/local/lib/python2.7/dist-packages/couchdbkit/client.py", line 513, in save_doc; doc1.update({'_id': res['id'], '_rev': res['rev']});KeyError: 'id';
```

The code lines in question are 510ff in client.py:

```
if 'batch' in params and 'id' in res:
doc1.update({ '_id': res['id']})
else:
doc1.update({'_id': res['id'], '_rev': res['rev']})
```

Obviously, the `'id' in res` check is useless since in the else case it's still being accessed directly from the response dict.