Add explanation of url encoding

elasticsearch/connection/http_urllib3.py

@@ -40,8 +40,12 @@ class Urllib3HttpConnection(Connection):
             kw = {}
             if timeout:
                 kw['timeout'] = timeout
+
+            # in python2 we need to make sure the url is not unicode. Otherwise
+            # the body will be decoded into unicode too and that will fail (#133).
             if not isinstance(url, str):
                 url = url.encode('utf-8')
+
             response = self.pool.urlopen(method, url, body, retries=False, headers=self.headers, **kw)
             duration = time.time() - start
             raw_data = response.data.decode('utf-8')