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

As of libxml 2.12.0, the `page.encoding` value inspected by this test
is dependent on how libiconv is built (specifically: which aliases are
supported).

- if alias "UTF8" is defined, then the page encoding will be "UTF-8"
- if alias "UTF8" is not defined, then the page encoding will be "UTF8"

The modified test started failing on Windows and MacOS (where Nokogiri
compiles libiconv and statically links it into the
native (precompiled) gem) but still passed on Linux (where libc
dynamically provides libiconv symbols).

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