Satellite 6 API call fails with 'Content-Type: ' is unsupported

Issue

PUT request to the API fails and receives this message from Red Hat Satellite 6:

"error": {"message":"'Content-Type: ' is unsupported in API v2 for POST and PUT requests. Please use 'Content-Type: application/json'."}

This happens despite correctly adding "Content-Type: application/json" to the PUT request's header.