code: def _assemble_headers(self, method, user_headers=None):
          
  
          headers = copy.deepcopy(user_headers or {})
  
          if method not in ('GET', 'HEAD'):
              headers.setdefault('Content-Type', 'application/json')
  
          return headers
docs: Takes the supplied headers and adds in any which
          are defined at a client level and then returns
          the result.
  
          :param user_headers: a `dict` containing headers defined at the
                               request level, optional.
  
          :return: a `dict` instance