Do not fail if client certificate is not provided

[borisreitman]

I should be able to request client certificate, but still allow the connection if the certificate is not provided.
In Node.js I can have the desired behaviour by specifying these options to HTTPS server,

requestCert: true,
rejectUnauthorized: false,

[Lukasa]

What are you trying to achieve in this usage configuration?

[borisreitman]

From an article referenced below, here is a quote:

"Then we tell it to accept requests with no valid certificate. We need this to handle invalid connections as well (for example to display an error message), otherwise, they would just get a cryptic HTTPS error message from the browser (ERR_BAD_SSL_CLIENT_AUTH_CERT to be precise)"

Reference: https://medium.com/@sevcsik/authentication-using-https-client-certificates-3c9d270e8326

[borisreitman]

@Lukasa
I have implemented it, please review my pull request: #413