Update PrimitiveFieldType.java

Add support of UTF-8!
For i18n (eg. TTS).

"A string must always contain UTF-8 encoded or 7-bit ASCII text." https://developers.google.com/protocol-buffers/docs/proto#scalar

"unicode strings are currently not supported as a ROS data type. utf-8 should be used to be compatible with ROS string serialization. " http://wiki.ros.org/msg

but it works. (testing with python)