[core] Use Enum.toString() in XmlStringBuilder.attribute(String, Enum<?>)

All other enum-using methods of XmlStringBuilder already use
Enum.toString(), as opposed to Enum.name(), this was the only left. I
do not remember why I did not to change this method too, probably
because of its plenty call sites.

But since this method already broke Jingle XML serializaton,
JingleAction was e.g., 'session_accept' when it should be
'session-accept', we change it now.

Fixes SMACK-921.

smack-core/src/main/java/org/jivesoftware/smack/util/XmlStringBuilder.java

@@ -289,8 +289,7 @@ public class XmlStringBuilder implements Appendable, CharSequence, Element {
 
     public XmlStringBuilder attribute(String name, Enum<?> value) {
         assert value != null;
-        // TODO: Should use toString() instead of name().
-        attribute(name, value.name());
+        attribute(name, value.toString());
         return this;
     }