Per https://www.openssl.org/docs/crypto/pem.html, specifying to encrypt
a private key with DES *without* providing a passphrase causes OpenSSL
to prompt for the passphrase to use on the console -- which is
problematic for GUI applications which generally lack a console.
This modifies the behavior of RSAKeyImpl::save() so that DES encryption
of private keys will not be attempted unless the passphrase argument is
non-empty. This will also suppress OpenSSL's prompting for that
passphrase on the console.
Also added a case to the test suite.
