Apply refinements and fixes to the MAM code

SMACK-625.

smack-extensions/src/main/java/org/jivesoftware/smackx/forward/provider/ForwardedProvider.java

@@ -36,6 +36,8 @@ import org.xmlpull.v1.XmlPullParser;
  */
 public class ForwardedProvider extends ExtensionElementProvider<Forwarded> {
 
+    public static final ForwardedProvider INSTANCE = new ForwardedProvider();
+
     private static final Logger LOGGER = Logger.getLogger(ForwardedProvider.class.getName());
 
     @Override

smack-extensions/src/main/java/org/jivesoftware/smackx/rsm/provider/RSMSetProvider.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014 Florian Schmaus
+ * Copyright © 2014-2016 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,8 @@ import org.xmlpull.v1.XmlPullParserException;
 
 public class RSMSetProvider extends ExtensionElementProvider<RSMSet> {
 
+    public static final RSMSetProvider INSTANCE = new RSMSetProvider();
+
     @Override
     public RSMSet parse(XmlPullParser parser, int initialDepth)
                     throws XmlPullParserException, IOException {

smack-extensions/src/main/java/org/jivesoftware/smackx/xdata/provider/DataFormProvider.java

@@ -40,6 +40,8 @@ import java.util.List;
  */
 public class DataFormProvider extends ExtensionElementProvider<DataForm> {
 
+    public static final DataFormProvider INSTANCE = new DataFormProvider();
+
     @Override
     public DataForm parse(XmlPullParser parser, int initialDepth) throws
                     Exception {