Fix MessagePackFactory.snapshot() to return a copy instead of this

Author: komamitsu

msgpack-jackson3/src/main/java/org/msgpack/jackson/dataformat/MessagePackFactory.java

@@ -166,7 +166,7 @@ public TokenStreamFactory copy()
     @Override
     public TokenStreamFactory snapshot()
     {
-        return this;
+        return copy();
     }
 
     @Override