(cs) Avoid redundant TreeMap.containsKey call in AbstractCharsetProvider

The method sun.nio.cs.ext.AbstractCharsetProvider#put is effectively equivalent of Map.putIfAbsent call.

    private static <K,V> void put(Map<K,V> m, K name, V value) {
        if (!m.containsKey(name))
            m.put(name, value);
    }

Instead of hand-written method we can use 'putIfAbsent' directly.
I makes code cleaner and gives a bit of performance.