add support for str8 packer

[reezhu]

Hi, thanks for you work
Recently I have meet a situation to communicate with msgpack in python.
The python project(can't modify) send the message with .packb(data, use_bin_type=True) and decode with .unpack(data, raw=False), but i found the java version seems to miss str8 encode support or do I miss the way to send a message with java with use_bin_type=True?

[xerial]

msgpack-java supports str8. Make sure you are using the latest version of msgpack-java.