XmlStringBuilder: Declare actual return type instead of supertype

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

@@ -627,7 +627,7 @@ public class XmlStringBuilder implements Appendable, CharSequence, Element {
     }
 
     @Override
-    public CharSequence toXML(XmlEnvironment enclosingXmlEnvironment) {
+    public StringBuilder toXML(XmlEnvironment enclosingXmlEnvironment) {
         // This is only the potential length, since the actual length depends on the given XmlEnvironment.
         int potentialLength = length();
         StringBuilder res = new StringBuilder(potentialLength);