Fix ineffective mount operation

HTTPAdatper mounted onto requests.Session using the 'http' prefix which is always superseded by already existing 'https://'.

Effectively, the current code will not retry the mixpanel POST request and will cause a connection reset error.

mixpanel/__init__.py

@@ -572,7 +572,7 @@ def __init__(self, events_url=None, people_url=None, import_url=None,
         )
 
         self._session = requests.Session()
-        self._session.mount('http', adapter)
+        self._session.mount('https://', adapter)
 
     def send(self, endpoint, json_message, api_key=None, api_secret=None):
         """Immediately record an event or a profile update.