Add upgrade handler to fix WebSocketBehavior template deduction on old examples (#1131)

examples/BroadcastingEchoServer.cpp

@@ -16,6 +16,7 @@ int main() {
         .idleTimeout = 10,
         .maxBackpressure = 1 * 1024 * 1024,
         /* Handlers */
+        .upgrade = nullptr,
         .open = [](auto *ws) {
             /* Let's make every connection subscribe to the "broadcast" topic */
             ws->subscribe("broadcast");

examples/EchoServer.cpp

@@ -24,6 +24,7 @@ int main() {
         .idleTimeout = 10,
         .maxBackpressure = 1 * 1024 * 1024,
         /* Handlers */
+        .upgrade = nullptr,
         .open = [](auto *ws) {
             /* Open event here, you may access ws->getUserData() which points to a PerSocketData struct */
         },

examples/EchoServerThreaded.cpp

@@ -22,6 +22,7 @@ int main() {
                 .idleTimeout = 10,
                 .maxBackpressure = 1 * 1024 * 1024,
                 /* Handlers */
+                .upgrade = nullptr,
                 .open = [](auto *ws) {
 
                 },