Fixed the example for bounce
Fixed the example for bounce

Used unix timestamp for the start_time and end_time in the bounce example.

Fixes #370

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