fix: escape foreign style tag content when serializing HTML5

Normally, a `style` tag is considered to be a raw text element,
meaning `<` is parsed as part of a possible "tag start" token, and is
serialized literally (and not rendered as an escaped character
reference `&lt;`).

However, when appearing in either SVG or MathML foreign content, a
`style` tag should *not* be considered a raw text element, and should
be escaped when serialized. libgumbo is parsing this case correctly,
but our HTML5 serialization code does not escape the content.

This commit updates the static `is_one_of()` C function to consider
the namespace of the parent node as well as the tag's local name when
deciding whether the tag matches the list of HTML elements, so that a
`style` tag in foreign content will *not* match, but a `style` tag in
HTML content will match.

(cherry picked from commit 44e3a74aff2c93873c82d55db8f08912f4e69d59)

ext/nokogiri/xml_node.c

@@ -1849,13 +1849,19 @@ is_one_of(xmlNodePtr node, char const *const *tagnames, size_t num_tagnames)
   if (name == NULL) { // fragments don't have a name
     return false;
   }
+
+  if (node->ns != NULL) {
+    // if the node has a namespace, it's in a foreign context and is not one of the HTML tags we're
+    // matching against.
+    return false;
+  }
+
   for (size_t idx = 0; idx < num_tagnames; ++idx) {
     if (!strcmp(name, tagnames[idx])) {
       return true;
     }
   }
   return false;
-
 }
 
 static void

test/html5/test_serialize.rb

@@ -553,4 +553,20 @@ def test_serializing_html5_fragment
     refute(fragment.send(:prepend_newline?))
     assert_equal("<div>hello</div>goodbye", fragment.to_html)
   end
+
+  describe "foreign content style tag serialization is escaped" do
+    it "with svg parent" do
+      input = %{<svg><style>&lt;img src>}
+      expected = %{<svg><style>&lt;img src&gt;</style></svg>}
+
+      assert_equal(expected, Nokogiri::HTML5.fragment(input).to_html)
+    end
+
+    it "with math parent" do
+      input = %{<math><style>&lt;img src>}
+      expected = %{<math><style>&lt;img src&gt;</style></math>}
+
+      assert_equal(expected, Nokogiri::HTML5.fragment(input).to_html)
+    end
+  end
 end if Nokogiri.uses_gumbo?