Encoding API: Update tests and simplify constructors

LayoutTests/fast/encoding/api/ascii-supersets-expected.txt

Index: LayoutTests/fast/encoding/api/ascii-supersets-expected.txt
diff --git a/LayoutTests/fast/encoding/api/ascii-supersets-expected.txt b/LayoutTests/fast/encoding/api/ascii-supersets-expected.txt
index 45071db96afe9263609979dbf9fc0d9f76b8529b..c2259de6a6a8fdced44f580847f816fdc4d3c35e 100644
--- a/LayoutTests/fast/encoding/api/ascii-supersets-expected.txt
+++ b/LayoutTests/fast/encoding/api/ascii-supersets-expected.txt
@@ -28,13 +28,11 @@ PASS ASCII superset encoding: windows-1258
 PASS ASCII superset encoding: x-mac-cyrillic 
 PASS ASCII superset encoding: gbk 
 PASS ASCII superset encoding: gb18030 
-PASS ASCII superset encoding: hz-gb-2312 
 PASS ASCII superset encoding: big5 
 PASS ASCII superset encoding: euc-jp 
 FAIL ASCII superset encoding: iso-2022-jp assert_equals: expected "\0\x01\x02\x03\x04\x05\x06\x07\b\t\n\v\f\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1c\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~" but got "\0\x01\x02\x03\x04\x05\x06\x07\b\t\n\v\f\r��\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1c\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~"
 FAIL ASCII superset encoding: shift_jis assert_equals: expected "\0\x01\x02\x03\x04\x05\x06\x07\b\t\n\v\f\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~" but got "\0\x01\x02\x03\x04\x05\x06\x07\b\t\n\v\f\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1c\x1b\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\x1a"
 PASS ASCII superset encoding: euc-kr 
-PASS ASCII superset encoding: iso-2022-kr 
 PASS ASCII superset encoding: x-user-defined 
 Harness: the test ran to completion.