bugfixes and emprovements on message creation from events

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

core/src/main/java/de/srsoftware/umbrella/core/model/Envelope.java

@@ -10,6 +10,7 @@ import static java.text.MessageFormat.format;
 
 import de.srsoftware.umbrella.core.exceptions.UmbrellaException;
 import java.time.LocalDateTime;
+import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
@@ -26,9 +27,9 @@ public class Envelope<T extends Message<?>> {
 		this(message,new HashSet<>(Set.of(receiver)));
 	}
 
-	public Envelope(T message, HashSet<User> receivers) {
+	public Envelope(T message, Collection<? extends User> receivers) {
 		this.message = message;
-		this.receivers = receivers;
+		this.receivers = new HashSet<>(receivers);
 		time = LocalDateTime.now();
 	}