UnicodeDecodeError when self._buffer table contains an unicode value

If msg is an unicode string here: https://github.com/djc/couchdb-python/blob/master/couchdb/http.py#L66
Then the CouchDB backend fails with UnicodeDecodeError here: https://github.com/djc/couchdb-python/blob/master/couchdb/http.py#L69