Fix mutable in signature

hbmqtt/client.py

@@ -135,7 +135,7 @@ class MQTTClient:
         cafile=None,
         capath=None,
         cadata=None,
-        extra_headers={},
+        extra_headers=None,
     ):
         """
         Connect to a remote broker.
@@ -154,6 +154,9 @@ class MQTTClient:
         :raise: :class:`hbmqtt.client.ConnectException` if connection fails
         """
 
+        if extra_headers is None:
+            extra_headers = {}
+
         self.session = self._initsession(uri, cleansession, cafile, capath, cadata)
         self.extra_headers = extra_headers
         self.logger.debug("Connect to: %s" % uri)