Support unsigned char in buffer

[drewdzzz]

While writing an example of working with raw data, I faced an interesting problem. I tried pack an empty MsgPack array manually (char data[] = {0x90};) but compiler suggested me to use unsigned char instead. It seemed to be a good idea but when I passed the result to the encoder, it failed to write to the buffer - the problem is encoder uses WData, which is, actually, a char * (the same story with RData, by the way), so compiler failed to find an appropriate overload of Buffer::write. We should inspect places using char * and fix them.