Improved support for Android and Chrome OS CP detection

Improved the support for Android and Chrome OS “Captive
Portal”detection. By returning a correct 204 status page when
/generate_204 is requested we prevent the device from disabling the
Wifi and roaming over to mobile data.

WiFiManager.cpp

@@ -46,7 +46,7 @@ void WiFiManager::begin(char const *apName) {
   server.on("/", std::bind(&WiFiManager::handleRoot, this));
   server.on("/wifi", std::bind(&WiFiManager::handleWifi, this));
   server.on("/wifisave", std::bind(&WiFiManager::handleWifiSave, this));
-  server.on("/generate_204", std::bind(&WiFiManager::handleRoot, this));  //Android captive portal. Maybe not needed. Might be handled by notFound handler.
+  server.on("/generate_204", std::bind(&WiFiManager::handle204, this));  //Android/Chrome OS captive portal check.
   server.on("/fwlink", std::bind(&WiFiManager::handleRoot, this));  //Microsoft captive portal. Maybe not needed. Might be handled by notFound handler.
   server.onNotFound (std::bind(&WiFiManager::handleNotFound, this));
   server.begin(); // Web server start
@@ -538,6 +538,15 @@ void WiFiManager::handleWifiSave() {
   connect = true; //signal ready to connect/reset
 }
 
+void WiFiManager::handle204() {
+  DEBUG_PRINT("204 No Response");  
+  server.sendHeader("Cache-Control", "no-cache, no-store, must-revalidate");
+  server.sendHeader("Pragma", "no-cache");
+  server.sendHeader("Expires", "-1");
+  server.send ( 204, "text/plain", "");
+}
+
+
 void WiFiManager::handleNotFound() {
   if (captivePortal()) { // If captive portal redirect instead of displaying the error page.
     return;

WiFiManager.h

@@ -94,6 +94,7 @@ private:
     void handleWifi();
     void handleWifiSave();
     void handleNotFound();
+    void handle204();
     boolean captivePortal();
     
     // DNS server