[ruby/cgi] Add test for escapeHTML/unescapeHTML invalid encoding fix in pure ruby version

Also, remove pointless assert_nothing_raised(ArgumentError) while
here.

https://github.com/ruby/cgi/commit/c05edf5608

test/cgi/test_cgi_util.rb

@@ -36,9 +36,7 @@ class CGIUtilTest < Test::Unit::TestCase
   end
 
   def test_cgi_escape_with_invalid_byte_sequence
-    assert_nothing_raised(ArgumentError) do
+    assert_equal('%C0%3C%3C', CGI.escape("\xC0\<\<".dup.force_encoding("UTF-8")))
-      assert_equal('%C0%3C%3C', CGI.escape("\xC0\<\<".dup.force_encoding("UTF-8")))
-    end
   end
 
   def test_cgi_escape_preserve_encoding
@@ -191,3 +189,32 @@ class CGIUtilTest < Test::Unit::TestCase
     assert_equal('&lt;BR&gt;<A HREF="url"></A>', unescape_element(escapeHTML('<BR><A HREF="url"></A>'), ["A", "IMG"]))
   end
 end
+
+class CGIUtilPureRubyTest < Test::Unit::TestCase
+  def setup
+    CGI::Escape.module_eval do
+      alias _escapeHTML escapeHTML
+      remove_method :escapeHTML
+      alias _unescapeHTML unescapeHTML
+      remove_method :unescapeHTML
+    end
+  end
+
+  def teardown
+    CGI::Escape.module_eval do
+      alias escapeHTML _escapeHTML
+      remove_method :_escapeHTML
+      alias unescapeHTML _unescapeHTML
+      remove_method :_unescapeHTML
+    end
+  end
+
+  def test_cgi_escapeHTML_with_invalid_byte_sequence
+    assert_equal("&lt;\xA4??&gt;", CGI.escapeHTML(%[<\xA4??>]))
+  end
+
+  def test_cgi_unescapeHTML_with_invalid_byte_sequence
+    input = "\xFF&"
+    assert_equal(input, CGI.unescapeHTML(input))
+  end
+end