A way to get USE_SSL attribute from listener

Hi,

There is any way to know if a listener is using SSL? I can get the port and host, but not "use ssl" option from listener.

```
int32_t HttpServerImpl::getSocketPort()
{
return drogon::app().getListeners()[0].toPort();
}

std::string HttpServerImpl::getSocketHost()
{
return drogon::app().getListeners()[0].toIp();
}
```

Thanks.