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.

def egress(self, envelope, http_headers, operation, binding_options):
    """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.
    """
    custom_headers = self._header_handler.GetHTTPHeaders()
    http_headers.update(custom_headers)
    return envelope, http_headers