Update challenges.py (CTFd#2564)

* Update challenges.py

Hi, i add this to solve a problem i detected  when POST to  /api/v1/challenges/attempt  content-type include charset -> 'content-type: application/json; charset=utf-8'.

I hope it helps!

* Update challenges.py

Modified to use Flask is_json as @ColdHeat suggested in CTFd#2564

Author: einar-lanfranco

CTFd/api/v1/challenges.py

@@ -497,7 +497,7 @@ def post(self):
         if authed() is False:
             return {"success": True, "data": {"status": "authentication_required"}}, 403
 
-        if request.content_type != "application/json":
+        if not request.is_json:
             request_data = request.form
         else:
             request_data = request.get_json()