HTTP error detection

The code (near [line 232](https://github.com/felixge/node-couchdb/blob/master/lib/couchdb.js#L232)) that detects CouchDB HTTP errors seems dangerous to me:

``` javascript
if ('error' in json) {
cbFired = true;
return cb && cb(json);
}
```

If I save a document with an 'error' attribute, node-couchdb will raise an error every time I fetch it. You should rely on the HTTP status code instead.