Add a test for SanitizationFilter::ANCHOR_SCHEMES

Clients may come to rely on this, so we should have a test to ensure we
don't break them.

Author: aroben

test/html/pipeline/sanitization_filter_test.rb

@@ -88,6 +88,10 @@ def test_whitelist_from_full_constant
     assert_equal 'Wat is this', html
   end
 
+  def test_exports_default_anchor_schemes
+    assert_equal SanitizationFilter::ANCHOR_SCHEMES, ['http', 'https', 'mailto', :relative, 'github-windows', 'github-mac']
+  end
+
   def test_script_contents_are_removed
     orig = '<script>JavaScript!</script>'
     assert_equal "", SanitizationFilter.call(orig).to_s