The recently added encodeASCII optimization is applicable to most DoubleByte encodings, which brings performance up to levels close to ISO-8859-1 for encoding ASCII-only char[]s.

Before:
Benchmark (size) (type) Mode Cnt Score Error Units
CharsetEncodeDecode.encode 16384 ISO-8859-1 avgt 30 3.021 ± 0.120 us/op
CharsetEncodeDecode.encode 16384 Shift-JIS avgt 30 47.793 ± 1.942 us/op
CharsetEncodeDecode.encode 16384 GB2312 avgt 30 49.598 ± 2.006 us/op
CharsetEncodeDecode.encode 16384 EUC-JP avgt 30 68.709 ± 5.019 us/op
CharsetEncodeDecode.encode 16384 EUC-KR avgt 30 48.033 ± 1.651 us/op

After:
Benchmark (size) (type) Mode Cnt Score Error Units
CharsetEncodeDecode.encode 16384 ISO-8859-1 avgt 30 2.856 ± 0.078 us/op
CharsetEncodeDecode.encode 16384 Shift-JIS avgt 30 5.287 ± 0.209 us/op
CharsetEncodeDecode.encode 16384 GB2312 avgt 30 5.490 ± 0.251 us/op
CharsetEncodeDecode.encode 16384 EUC-JP avgt 30 7.657 ± 0.368 us/op
CharsetEncodeDecode.encode 16384 EUC-KR avgt 30 5.718 ± 0.267 us/op