Django: unable to set campaign when using EmailMessage object

[andyzickler]

I'm trying to send an email with a campaign id using a EmailMessage object as follows:

msg = EmailMessage(
    subject='Test Campaign',
    to=['test@test.com']
)
msg.template = 'test-template'
msg.substitution_data = substitution_data
msg.campaign = 'test-campaign'
msg.send()

Viewing the message in the SparkPost Dashboard, I can see the template and data are set, but the campaign remains empty. Following #107, it looks like the problem is in the SparkPostMessage class. substitution_data, template are copied there, but not campaign.