envelope.go
func (e *Envelope) AddHeader(name string, value string) error {
	if name == "" {
		return fmt.Errorf("Provide non-empty header name")
	}

	e.header.Add(name, mime.BEncoding.Encode("utf-8", value))
	return nil
}
func (e *Envelope) AddHeader(name string, value string) error {
	if name == "" {
		return fmt.Errorf("Provide non-empty header name")
	}

	e.header.Add(name, mime.BEncoding.Encode("utf-8", value))
	return nil
}
// AddHeader appends given header value to header name without changing existing values.
// If the header does not exist already, it will be created.