Feature Request: Allow for refractive indices lower than 1

[Manuhart]

Some materials like metals and alloys have refractive indices that are lower than 1.0 but currently those cannot be used since Corona has a minimum of 1.

[AlexejK]

Corona uses the "simplified" model of fresnel without extinction coefficient. So values below 1 are nonsense in this case. Just use the higher values above 20 for metals. If you want to go more accurate, rebuild the complex fresnel curve like iNevestenko did here: https://forum.corona-renderer.com/index.php?topic=13034.0