fixed packExtendedTypeHeader for payload length = 32, 64, 128

@MartinWeindel 
In principle, MessagePack favors speed. For example, we have considered which is the fastest, using switch-case, if-else statements or table lookup? https://github.com/msgpack/msgpack-java/pull/85

In API level, we prefer readable code, but the implementation should be as fast as possible. In general, it might be 0.1% improvement, but If the user frequently creates variable length data of EXTn types, even such a tiny optimization might have a great impact. 

Thanks in advance.