NKF still uses Charset transcoding rather than jcodings

[headius]

There are four failures in test_kconv.rb that appear to be caused, at least in part, by our NKF library still using Java's Charset to do transcoding rather than jcodings. Many of the encodings that jcodings supports are not available in Java.

We should try to fix up the remaining NKF issues by using jcodings to do transcoding like MRI.

Relates to #3261.

[headius]

This is fixed in c6e43c0.