whisper: fix send timestamp omission during envelope opening

whisper/envelope.go

@@ -72,6 +72,7 @@ func (self *Envelope) Open(key *ecdsa.PrivateKey) (msg *Message, err error) {
 
 	message := &Message{
 		Flags: data[0],
+		Sent:  int64(self.Expiry - self.TTL),
 	}
 	data = data[1:]