Set the ServerName for TLS configuration (#988)

When TLS hostname validation used for the MySQL connection, the
ServerName property needs to be set so that it knows which name to
validate on the certificate.

Without this option and with InsecureSkipVerify set to false, validation
will error here with a fatal error otherwise:

```
FATAL tls: either ServerName or InsecureSkipVerify must be specified in the tls.Config
```

go/mysql/connection.go

@@ -92,6 +92,7 @@ func (this *ConnectionConfig) UseTLS(caCertificatePath, clientCertificate, clien
 	}
 
 	this.tlsConfig = &tls.Config{
+		ServerName:         this.Key.Hostname,
 		Certificates:       certs,
 		RootCAs:            rootCertPool,
 		InsecureSkipVerify: allowInsecure,