Bug in Arduino_LoRaWAN.h - kCountryCode()

```
        // return the country code
        constexpr uint16_t kCountryCode(char c1, char c2)
                {
                return (c1 < 'A' || c1 > 'Z' || c2 < 'A' || c2 > 'Z')
                        ? 0
                        : ((c1 << 8) | c1);
                }
```

The last part of the return value should probably read
```
((c1 << 8) | c2)
``` 