code: def egress(self, envelope, http_headers, operation, binding_options):
      
      custom_headers = self._header_handler.GetHTTPHeaders()
      http_headers.update(custom_headers)
      return envelope, http_headers
docs: Overriding the egress function to set our headers.
  
      Args:
        envelope: An Element with the SOAP request data.
        http_headers: A dict of the current http headers.
        operation: The SoapOperation instance.
        binding_options: An options dict for the SOAP binding.
  
      Returns:
        A tuple of the envelope and headers.