BCC shown in emails sent via SMTP

By design, if a contribution page is configured to Bcc receipts to a given address, the Bcc should not be included in emails to any recipient of that receipt.

This works find for mail(): the Bcc header is correctly not included in any receipt emails, if mail is sent with mail(), i.e., if "mail()" is selected at /civicrm/admin/setting/smtp.

But there's a problem with SMTP: The Bcc header IS visible in the email to the donor, if mail is sent with SMTP, i.e., if "SMTP" is selected at /civicrm/admin/setting/smtp.

PR coming soon.