RequestServer: Make value copy of the URL in ensure_connection()

I saw what looked like a UAF of this URL in a RequestServer crash,
and it seems reasonable to make a copy here since we end up passing
them to Core::deferred_invoke().

Userland/Services/RequestServer/ClientConnection.cpp

@@ -126,7 +126,7 @@ void ClientConnection::ensure_connection(URL const& url, ::RequestServer::CacheL
     }
 
     struct {
-        URL const& m_url;
+        URL m_url;
         void start(Core::Stream::Socket& socket)
         {
             auto is_connected = socket.is_open();