Unable to send with BCC only

[gjcourtney]

If I want to send an email to multiple recipients, where there is no "To" and all emails are delivered with BCC, the SDK generates a MissingRequiredParameter exception with the message "The parameters passed to the API were invalid. Please specify "to"".

This behaviour is unique to the SDK and is not enforced by the API, so I believe it to be a bug in the SDK. Are you willing to receive a PR that checks for the presence of "To" OR BCC recipients?