Add SAN field to the x509 certificate

[kenwer]

I was getting the following error when running the TLS example:

=== RUN   Example_tls
2020/09/29 13:48:13 http: TLS handshake error from [::1]:63266: remote error: tls: bad certificate
--- FAIL: Example_tls (0.94s)
got:
Ping failed: Http client get failed: Get "https://localhost:63265": x509: certificate relies on legacy Common Name field, use SANs or temporarily enable Common Name matching with GODEBUG=x509ignoreCN=0

This patch adds the SAN field to the x509 certificate so it doesn't rely just on legacy Common Name field to fix this error.