docs: def define_header_values(self, http_method, route, values, update=False):
          """Define header values for a given request.
  
          By default, header values are determined from the class attribute
          `headers`. But if you want to change the headers used in the
          documentation for a specific route, this method lets you do that.
  
          :param str http_method: An HTTP method, like "get".
          :param str route: The route to match.
          :param dict values: A dictionary of headers for the example request.
          :param bool update: If True, the values will be merged into the default
              headers for the request. If False, the values will replace
              the default headers.
          """
          self.defined_header_values[(http_method.lower(), route)] = {
              'update': update,
              'values': values
          }