Param amount format and prod mode fixes.

Problem/Motivation

The amount needs a different format, for example with the current version if your order amount is 50 you will get 0.50 instead of 50.0 on Nelnet side.

Add getSupportedModes on QuikpayRedirectCheckout the defaults are test, live and not test, prod

QuikpayRedirectCheckout onReturn method needs to validate transactionStatus instead of transactionResultCode, transactionResultCode is not present on prod mode.