Fix some bugs

- Fix a bug that `GET` requests without data will report an error in API
- Fix a bug that `aggregate` requests will get an error when the inner function raises an error
- Fix a bug that the charts of a course cannot be the same because of the incorrect primary keys
- Fix a bug that global ranking scores cannot be calculated if there are no chart in the database #61
- The first try to fix #60 (I don't think it can be done so easily.)

latest version/api/api_auth.py

@@ -16,7 +16,8 @@ def role_required(request, powers=[]):
         @wraps(view)
         def wrapped_view(*args, **kwargs):
             try:
-                request.json  # 检查请求json格式
+                if request.data:
+                    request.json  # 检查请求json格式
             except:
                 return error_return(PostError('Payload must be a valid json', api_error_code=-1), 400)