'Make sure we can manually set the To header (#17444)'

def test_to_header(self):

    email = EmailMessage(u'Subject', u'Content', u'bounce@example.com', [u'list-subscriber@example.com', u'list-subscriber2@example.com'], headers={u'To': u'mailing-list@example.com'})
    message = email.message()
    self.assertEqual(message[u'To'], u'mailing-list@example.com')
    self.assertEqual(email.to, [u'list-subscriber@example.com', u'list-subscriber2@example.com'])
    email = EmailMessage(u'Subject', u'Content', u'bounce@example.com', [u'list-subscriber@example.com', u'list-subscriber2@example.com'])
    message = email.message()
    self.assertEqual(message[u'To'], u'list-subscriber@example.com,    list-subscriber2@example.com')
    self.assertEqual(email.to, [u'list-subscriber@example.com', u'list-subscriber2@example.com'])
