wrap WifiSecureClient by default

[proppy]

Using Arduino Client interface like it was originally before d72b3ff.

This could make the API a lot easier Serial.print(Firebase.push(...)), but we still need a way to perform operation in a non blocking way while leaving tight control on the arduino main loop, ex:

FirebaseReq req = Firebase.child("logs").push("{\".sv\": \"timestamp\"}");
while (!req.done()) {
   req.transfer();
   // blink some led while transferring
   digitalWrite(LED_PIN, !digitalRead(LED_PIN));
}
Serial.print(req.response());

We also need a way to dump the intermediate request headers for debugging and offer a way to stream the request body.