In scepclient/scepclient.go, would it make sense to use application/x-pki-message instead of application/octet-stream for Content-Type? Or add an optional parameter that allows the user to specify the content-type header?

(I don't mind making the PR myself. But let me know if this is an acceptable direction.)

My MDM server expects application/x-pki-message for the PKIOperation request.