test: accept "UTF-8" as an encoding alias for "UTF8"

as of libxml 2.12.0, the result of this test is dependent on how
libiconv is built (which aliases are supported), and it started
failing on windows and macos (but strangely, not linux).

if the alias "UTF8" is defined, then the result will be "UTF-8".
if the alias "UTF8" is not defined, then the result will be "UTF8".

test/test_mechanize_http_agent.rb

@@ -1309,8 +1309,13 @@ def test_response_parse_content_type_encoding_broken_utf_8
     page = @agent.response_parse @res, body, @uri
 
     assert_instance_of Mechanize::Page, page
-    assert_equal 'UTF8', page.encoding
-    assert_equal 'UTF8', page.parser.encoding
+
+    # as of libxml 2.12.0, the result is dependent on how libiconv is built (which aliases are supported)
+    # if the alias "UTF8" is defined, then the result will be "UTF-8".
+    # if the alias "UTF8" is not defined, then the result will be "UTF8".
+    # note that this alias may be defined by Nokogiri itself in its EncodingHandler class.
+    assert_includes ["UTF8", "UTF-8"], page.encoding
+    assert_includes ["UTF8", "UTF-8"], page.parser.encoding
   end
 
   def test_response_parse_content_type_encoding_garbage