.delete() causes a 400 error

[mhl]

The server replies with a 400 Bad Request error if you send a request with a Content-Type: application/json but no content. This is correct behaviour - a zero-length string isn't valid JSON. @evdb pointed out that there is an open pull request to fix this in slumber:

samgiles/slumber#32

We should fork slumber, add that patch, and point to our version in requirements.txt until it's fixed upstream.