Use SHA-512 instead of MD5

Hello AuthroizedNet Team,

Is there any update needed in this library to accommodate the following changes? 
I found that for generating the token, library calling **securePaymentContainerRequest** API. 

[MD5 Hash End of Life](https://support.authorize.net/s/article/MD5-Hash-End-of-Life-Signature-Key-Replacement#sample)
