Add SSLOPT to SIM800 with preprocessor flag to use

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>

examples/HttpsClient/HttpsClient.ino

@@ -71,6 +71,9 @@
 // set GSM PIN, if any
 #define GSM_PIN ""
 
+// flag to force SSL client authentication, if needed
+// #define TINY_GSM_SSL_CLIENT_AUTHENTICATION
+
 // Your GPRS credentials, if any
 const char apn[]  = "YourAPN";
 const char gprsUser[] = "";
@@ -81,9 +84,9 @@ const char wifiSSID[]  = "YourSSID";
 const char wifiPass[] = "YourWiFiPass";
 
 // Server details
-const char server[] = "vsh.pp.ua";
+const char server[]   = "vsh.pp.ua";
 const char resource[] = "/TinyGSM/logo.txt";
-const int  port = 443;
+const int  port       = 443;
 
 #include <TinyGsmClient.h>
 #include <ArduinoHttpClient.h>
@@ -103,15 +106,15 @@ const int  port = 443;
 #endif
 
 #ifdef DUMP_AT_COMMANDS
-  #include <StreamDebugger.h>
-  StreamDebugger debugger(SerialAT, SerialMon);
-  TinyGsm modem(debugger);
+#include <StreamDebugger.h>
+StreamDebugger debugger(SerialAT, SerialMon);
+TinyGsm        modem(debugger);
 #else
-  TinyGsm modem(SerialAT);
+TinyGsm modem(SerialAT);
 #endif
 
 TinyGsmClientSecure client(modem);
-HttpClient http(client, server, port);
+HttpClient          http(client, server, port);
 
 void setup() {
   // Set console baud rate

src/TinyGsmClientSIM800.h

@@ -506,6 +506,18 @@ class TinyGsmSim800 : public TinyGsmModem<TinyGsmSim800>,
     sendAT(GF("+CIPSSL="), ssl);
     rsp = waitResponse();
     if (ssl && rsp != 1) { return false; }
+#ifdef TINY_GSM_SSL_CLIENT_AUTHENTICATION
+    // set SSL options
+    // +SSLOPT=<opt>,<enable>
+    // <opt>
+    //    0 (default) ignore invalid certificate
+    //    1 client authentication
+    // <enable>
+    //    0 (default) close
+    //    1 open
+    sendAT(GF("+CIPSSL=1,1"));
+    if (waitResponse() != 1) return false;
+#endif
 #endif
     sendAT(GF("+CIPSTART="), mux, ',', GF("\"TCP"), GF("\",\""), host,
            GF("\","), port);