HTTP error detection

[lperrinfano]

The code (near line 232) that detects CouchDB HTTP errors seems dangerous to me:

          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.