v1.8.15-0 to permit sending data larger than 4K

### Releases v1.8.15-0

1. Fix severe limitation to permit sending much larger data than total 4K. Check [server.send buffer size limited to 4k #23](khoih-prog/WiFiWebServer#23)
2. Add examples [WiFiWebServer_BigData](https://github.com/khoih-prog/WiFiNINA_Generic/tree/master/examples/WiFiWebServer_BigData) to demo how to send much larger data than total 4K
3. Optimize code
4. Clean up