(cs) Encoding U+FFFE, U+FFFF should throw UnmappableCharacterException

The U+FFFF character should cause a java.nio.charset.MalformaledInputException to be thrown by the CharsetEncoder encode() method.
But now it passes the encode, doesn't throw any exception.
During API changes, now U+FFFE & U+FFFF return CoderResult.isUnmappable().