title: Client may not trust proxy certificate although add_upstream_certs_to_client_chain is set to True
body: <p>I'm trying to perform MITM between a remote server and IoT internet bridge device but even though I set add_upstream_certs_to_client_chain=true, I'm still getting 'The client may not trust the proxy's certificate' error so basically it means proxy does not use upstream certificate against client.</p>
  <p>This is how I run mitmproxy:</p>
  <p>--mode transparent --ssl-insecure --set add_upstream_certs_to_client_chain=true</p>
  <p>what am I doing wrong?</p>