fixed timestamp for first message when combining multiple

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

messages/src/main/java/de/srsoftware/umbrella/message/model/CombinedMessage.java

@@ -57,7 +57,7 @@ public class CombinedMessage {
 				combinedSubject = subject;
 				break;
 			case 1:
-				combinedBody.insert(0,format("# {0} @ {1}\n→ {2}:\n\n",sender, dateTimeOf(message.utcTime()),subject)); // insert sender and subject of first message right before the body of the first message
+				combinedBody.insert(0,format("# {0} @ {1}\n→ {2}:\n\n",sender, dateTimeOf(mergedMessages.getFirst().utcTime()),subject)); // insert sender and subject of first message right before the body of the first message
 				combinedSubject = subjectForCombinedMessage.translate(lang);
 				// no break here, we need to append the subject and content
 			default: