D8: Add support for CC and BCC recipients

Problem/Motivation

Though the Mandrill API does appear to support the addition of CC and BCC recipients, this functionality is not incorporated into the Mandrill module.

See the following excerpt from the Mandrill API documentation:

From: https://mandrillapp.com/api/docs/messages.html

Proposed resolution

In MandrillMail.php, the following lines could be updated to incorporate CC and BCC recipients from mail headers:

    // Extract an array of recipients.
    $to = $this->mandrill->getReceivers($message['to']);

And the following function from MandrillService.php could be updated to apply the 'type' setting as needed to classify emails as either 'to', 'cc', or 'bcc'.

  /**
   * Helper to generate an array of recipients.
   *
   * @param mixed $receiver
   *   a comma delimited list of email addresses in 1 of 2 forms:
   *   user@domain.com
   *   any number of names <user@domain.com>
   *
   * @return array
   *   array of email addresses
   */
  public function getReceivers($receiver) {
    $recipients = array();
    $receiver_array = explode(',', $receiver);
    foreach ($receiver_array as $email) {
      if (preg_match(MANDRILL_EMAIL_REGEX, $email, $matches)) {
        $recipients[] = array(
          'email' => $matches[2],
          'name' => $matches[1],
        );
      }
      else {
        $recipients[] = array('email' => $email);
      }
    }
    return $recipients;
  }

Remaining tasks

Write the code and upload a patch for review and testing.

User interface changes

None.

API changes

None. Just that it would add support for CC and BCC addresses which Mandrill already has.

Data model changes

The $receiver variable in getReceivers would be either an string or an array. Which would have to be accounted for in the update. The update should provide a check of some kind to provide backward compatibility for string inputs.