func (e *Envelope) GetHeaderKeys() (headers []string) {
	if e.header == nil {
		return
	}
	for key := range *e.header {
		headers = append(headers, key)
	}
	return headers
}
func (e *Envelope) GetHeaderKeys() (headers []string) {
	if e.header == nil {
		return
	}
	for key := range *e.header {
		headers = append(headers, key)
	}
	return headers
}
func ( e * Envelope ) GetHeaderKeys ( ) ( headers [ ] string ) { if e . header == nil { return } for key := range * e . header { headers = append ( headers , key ) } return headers }
// GetHeaderKeys returns a list of header keys seen in this message. Get
// individual headers with `GetHeader(name)`
GetHeaderKeys returns a list of header keys seen in this message . Get individual headers with GetHeader ( name )