GSM: add isConnected() method

libraries/GSM/src/GSM.cpp

@@ -130,6 +130,15 @@ bool arduino::GSMClass::setTime(unsigned long const epoch, int const timezone)
   return _device->set_time(epoch, timezone);
 }
 
+bool arduino::GSMClass::isConnected()
+{
+  if (_context) {
+    return _context->is_connected();
+  } else {
+    return false;
+  }
+}
+
 static PlatformMutex trace_mutex;
 
 static void trace_wait()

libraries/GSM/src/GSM.h

@@ -91,6 +91,7 @@ class GSMClass : public MbedSocketClass {
   int ping(const char* hostname, uint8_t ttl = 128);
   int ping(const String& hostname, uint8_t ttl = 128);
   int ping(IPAddress host, uint8_t ttl = 128);
+  bool isConnected();
 
   friend class GSMClient;
   friend class GSMUDP;