Issue Summary

Setting the sandbox mode to false triggers this error: https://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/errors.html#message.mail_settings.sandbox_mode.enable because it generates an empty object for sandbox_mode key in the API request body,

Steps to Reproduce

1. Create a new MailSettings and call setSandboxMode on a Setting with enabled
2. Set the MailSettings on your new Mail
3. Send the email

You can reproduce the problem by modifying one of the tests like this: https://github.com/ludovicofischer/sendgrid-java/commit/4987e9f06d535690459d3e939098d6b246e1f7fe

Technical details:

• sendgrid-java Version: master (latest commit: [9d14b95])
• Java Version: 1.8