Attempts to sign the content MIME part with userid's private key
using the ctx signing context. If successful, a new multipart/signed
object is returned.
a new #GMimeMultipartSigned object on success
or null on fail. If signing fails, an exception will be set on err to provide
information as to why the failure occurred.
Attempts to sign the content MIME part with userid's private key using the ctx signing context. If successful, a new multipart/signed object is returned.