set_tls_client_certificate
Description:
Sets the certificate to be used by this's connection when a client certificate is requested 
      during the TLS handshake.
You can call this as a response to [signal@Message:SoupMessage:request-certificate] signal, or before the connection is started. 
      If certificate is null the handshake will continue without providing a GTlsCertificate. Note that
       the [class@Gio.TlsCertificate] set by this function will be ignored if [property@Session:tls-interaction] is not 
        null.
Parameters:
| this | a Message | 
| certificate | the TlsCertificate to set, or null |