JNI UTF Char Manipulation Bugs

[dzagieboylo]

Some of the JNI methods related to UTF characters are only partially implemented.

E.g. jni_GetStringUTFChars only works when the argument Java String only consists of UTF-8 characters (even though it supports UTF-16 characters too).

jni_NewStringUTF is also buggy, working only with a subset of Java String characters.