A call to create a mailing list responds with text/html
content type.
~ → curl -i -s --user 'api:key-...' --data 'name=TestCampaign' -X POST 'https://api.mailgun.net/v2/sandbox67449.mailgun.org/campaigns'
HTTP/1.1 200 OK
Server: nginx/1.4.1
Date: Fri, 28 Mar 2014 14:27:26 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 302
Connection: keep-alive