LibWeb: Fix crash in get_response_mime_type()

We were creating a ref-counted HeaderList object on the stack,
which then crashed in ~RefCounted() on scope exit, since nobody had
adopted it.

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -215,17 +215,17 @@ MimeSniff::MimeType XMLHttpRequest::get_final_mime_type() const
 MimeSniff::MimeType XMLHttpRequest::get_response_mime_type() const
 {
     // FIXME: Use an actual HeaderList for XHR headers.
-    Fetch::Infrastructure::HeaderList header_list;
+    auto header_list = make_ref_counted<Fetch::Infrastructure::HeaderList>();
     for (auto const& entry : m_response_headers) {
         auto header = Fetch::Infrastructure::Header {
             .name = MUST(ByteBuffer::copy(entry.key.bytes())),
             .value = MUST(ByteBuffer::copy(entry.value.bytes())),
         };
-        MUST(header_list.append(move(header)));
+        MUST(header_list->append(move(header)));
     }
 
     // 1. Let mimeType be the result of extracting a MIME type from xhr’s response’s header list.
-    auto mime_type = header_list.extract_mime_type();
+    auto mime_type = header_list->extract_mime_type();
 
     // 2. If mimeType is failure, then set mimeType to text/xml.
     if (!mime_type.has_value())