Fix failing test case on musl based Linux

Musl doesn't recognize cp932 encoding.
It also fails when used on the command line with iconv.
For this test case cp932 can be replaced by Shift_JIS, which is identical for the characters in question and that is handled by musl.

Error was:

  1) Error:
Nokogiri::HTML::TestDocumentEncoding#test_encoding_without_charset:
ArgumentError: invalid byte sequence in UTF-8
    /home/lars/comcard/nokogiri/test/html/test_document_encoding.rb:26:in `test_encoding_without_charset'

Related to #1983

test/html/test_document_encoding.rb

@@ -19,7 +19,7 @@ def test_encoding
       end
 
       def test_encoding_without_charset
-        doc = Nokogiri::HTML File.open(SHIFT_JIS_NO_CHARSET, 'r:cp932:cp932').read
+        doc = Nokogiri::HTML File.open(SHIFT_JIS_NO_CHARSET, 'r:Shift_JIS:Shift_JIS').read
 
         hello = "こんにちは"