The latest version of `rustls` acts on the SNI extension to TLS without
the apparent ability to disable the behavior. `rustls` requires that the
server's certificate match the client's requested server. The matching
is done by looking at DNS names in the `subjectAltName` extension and
checking if the requested server name is present. Since the certificate
in the `tls` example did not have the `subjectAltName` extension, this
check always failed, and the TLS connection was aborted. This commit
adds the extension to the certificate with a DNS name of `localhost`,
ensuring that TLS succeeds on `localhost`.
Sergio Benitez