Refactored, use HttpStatusCode enum for retriable errors and better naming
Refactored, use HttpStatusCode enum for retriable errors and better naming

sendgrid/sendgrid-csharp,sendgrid/sendgrid-csharp,sendgrid/sendgrid-csharp,dubrovkinmaxim/sendgrid-csharp,dubrovkinmaxim/sendgrid-csharp