GH-52: Fix "only one mechanism must be provided" issue

Author: ArtyomVancyan

src/fastapi_oauth2/core.py

@@ -125,8 +125,11 @@ async def token_data(self, request: Request, **httpx_client_args) -> dict:
         async with httpx.AsyncClient(auth=auth, **httpx_client_args) as session:
             try:
                 response = await session.post(token_url, headers=headers, content=content)
-                if response.status_code == 401:
-                    content = re.sub(r"client_id=[^&]+", "", content)
+                if response.is_error:
+                    if response.status_code == 401:
+                        content = re.sub(r"client_id=[^&]+", "", content)
+                    elif response.status_code == 400:
+                        content = re.sub(r"client_secret=[^&]+", "", content)
                     response = await session.post(token_url, headers=headers, content=content)
                 self._oauth_client.parse_request_body_response(json.dumps(response.json()))
                 return self.standardize(self.backend.user_data(self.access_token))