Improve MUCs subject changed notification filter

Filter also messages with bodies or thread element.

smack-core/src/main/java/org/jivesoftware/smack/filter/MessageWithThreadFilter.java

@@ -0,0 +1,43 @@
+/**
+ *
+ * Copyright 2017 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jivesoftware.smack.filter;
+
+import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smack.util.StringUtils;
+
+/**
+ * Filters message stanzas which have at least one body.
+ */
+public final class MessageWithThreadFilter extends FlexibleStanzaTypeFilter<Message> {
+
+    public static final StanzaFilter INSTANCE = new MessageWithThreadFilter();
+
+    private MessageWithThreadFilter() {
+        super(Message.class);
+    }
+
+    @Override
+    protected boolean acceptSpecific(Message message) {
+        return StringUtils.isNotEmpty(message.getThread());
+    }
+
+    @Override
+    public String toString() {
+        return getClass().getSimpleName();
+    }
+}