LibWeb: Allow cloneNode() to clone elements with weird attributes

We can't rely on Element.setAttribute() in cloneNode() since that will
throw on weird attribute names. Instead, just follow the spec and copy
attributes into cloned elements verbatim.

This fixes a crash when loading the "issues" tab on GitHub repos.
They are actually sending us unintentionally broken markup, but we
should still support cloning it. :^)

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -826,7 +826,7 @@ JS::NonnullGCPtr<Node> Node::clone_node(Document* document, bool clone_children)
         element.for_each_attribute([&](auto& name, auto& value) {
             // 1. Let copyAttribute be a clone of attribute.
             // 2. Append copyAttribute to copy.
-            MUST(element_copy->set_attribute(name, value));
+            element_copy->append_attribute(name, value);
         });
         copy = move(element_copy);