add the ability to load from file a DER based certificate

[marco-introini]

With this little improvement it's now possible to load a file in which the certificate used is in binary DER based format instead of classic PEM format.

[marco-introini]

I moved the der2pem function inside the SslCertificate class as a public static function.
Previous test didn't work for connection error on site with different port. I found another one (casually) which works for now and modified them.
Hope this can help

Thank you so much

[freekmurze]

Thanks!