code: def _user_headers(self, headers=None):
          
          h = self.copy()
  
          if headers is not None:
              keys = set(headers.keys())
              if h.get('Authorization', False):
                  keys -= {'Authorization'}
  
              for key in keys:
                  h[key] = headers[key]
  
          return h
docs: Make sure the user doesn't override the Authorization header