Make Message.setType(Type) not throw if type is null

RFC 6121 § 5.2.2:
"""
If an application receives a message with no 'type' attribute or the
application does not understand the value of the 'type' attribute
provided, it MUST consider the message to be of type "normal" (i.e.,
"normal" is the default).
"""

smack-core/src/test/java/org/jivesoftware/smack/packet/MessageTest.java

@@ -69,12 +69,6 @@ public class MessageTest {
         assertXMLEqual(control, messageTypeSet.toXML().toString());
     }
 
-    @Test(expected=IllegalArgumentException.class)
-    public void setMessageTypeNullTest() {
-        Message message = getNewMessage();
-        message.setType(null);
-    }
-
     @Test(expected=NullPointerException.class)
     public void setNullMessageBodyTest() {
         Message message = getNewMessage();