test: fix encoding in the unicode XSS test

See flavorjones/loofah#205 for a short history
of this test string.

Related to #111

test/sanitizer_test.rb

@@ -414,7 +414,7 @@ def test_should_sanitize_img_dynsrc_lowsrc
   end
 
   def test_should_sanitize_div_background_image_unicode_encoded
-    raw = %(background-image:\0075\0072\006C\0028'\006a\0061\0076\0061\0073\0063\0072\0069\0070\0074\003a\0061\006c\0065\0072\0074\0028.1027\0058.1053\0053\0027\0029'\0029)
+    raw = %(background-image:\u0075\u0072\u006C\u0028'\u006a\u0061\u0076\u0061\u0073\u0063\u0072\u0069\u0070\u0074\u003a\u0061\u006c\u0065\u0072\u0074\u0028.1027\u0058.1053\u0053\u0027\u0029'\u0029)
     assert_equal '', sanitize_css(raw)
   end