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

def test_to_header(self):

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