update client to support auth_token

support `auth_token` to fix #25

nats/io/client.py

@@ -289,8 +289,13 @@ def connect_command(self):
     }
     if "auth_required" in self._server_info:
       if self._server_info["auth_required"] == True:
-        options["user"] = self._current_server.uri.username
-        options["pass"] = self._current_server.uri.password
+        # In case there is no password, then consider handle
+        # sending a token instead.
+        if self._current_server.uri.password is None:
+          options["auth_token"] = self._current_server.uri.username
+        else:
+          options["user"] = self._current_server.uri.username
+          options["pass"] = self._current_server.uri.password
     if self.options["name"] is not None:
       options["name"] = self.options["name"]