LibWeb: Fix UAF in convert_header_names_to_a_sorted_lowercase_set()

We can't keep a span (ReadonlyBytes) to a move()'d ByteBuffer
in the header_names_seen HashTable - copy the original name span instead
which works the same thanks to CaseInsensitiveBytesTraits.

This would sporadically fail the contains() check due to garbage data,
later leading to a VERIFY() crash in the OrderedHashTable append loop.

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp

@@ -352,7 +352,7 @@ ErrorOr<OrderedHashTable<ByteBuffer>> convert_header_names_to_a_sorted_lowercase
             continue;
         auto bytes = TRY(ByteBuffer::copy(name));
         Infra::byte_lowercase(bytes);
-        header_names_seen.set(bytes);
+        header_names_seen.set(name);
         header_names_set.append(move(bytes));
     }