title: BCC in Mailgun Batch Send does not include substitutions
body: <p>I am sending a set of email messages via Mailgun using the Batch Sending feature of their API, with a call like this:</p>
  
  <pre><code>rv = requests.post(
      "https://api.mailgun.net/v3/%s/messages" % mailgun_domain,
      auth=("api", mailgun_key),
      data={"from": sender,
          "to": recip_vars.keys(),
          "subject": subject,
          "bcc": bcc_addr,
          "text": "%recipient.text%",
          "html": "%recipient.html%",
          "recipient-variables":json.dumps(recip_vars)})
  </code></pre>
  
  <p>where <code>recip_vars</code> is a dictionary of the batch variables keyed by recipient addresses.</p>
  
  <p>In the copy sent to the <code>bcc</code> address, the <code>recip_vars</code> substitution has not been made.</p>
  
  <p>Does the <code>bcc</code> address need to be specified in a different way in order for that copy to include the substitutions, and if so, how?</p>