Fix: use POST for continueAuthentication endpoint (#317)

Volvo's PingFederate auth API now requires a POST request for the
continueAuthentication step after OTP verification. Using GET returns
the same OTP_VERIFIED state in a loop, never progressing to COMPLETED,
which causes a KeyError on 'authorizeResponse'.

Changing auth_session.get() to auth_session.post() with an empty JSON
body causes the API to return COMPLETED with the expected authorizeResponse.

src/volvo.py

@@ -114,7 +114,7 @@ def authorize(renew_tokenfile=False):
 
 def continue_auth(auth_session, data):
     next_url = data["_links"]["continueAuthentication"]["href"].replace("http://", "https://") + "?action=continueAuthentication"
-    auth = auth_session.get(next_url)
+    auth = auth_session.post(next_url, data="{}")
 
     if auth.status_code == 200:
         return auth.json()