Only serialize public keys by default to prevent inadvertent leaking of private keys

[geigerzaehler]

At the moment it is quite easy to inadvertently leak a private key when using BaseKey.as_dict() or KeySet.as_dict(). These method return private key information by default. To prevent this I'd suggest changing the default of the private param for as_dict() to False instead of None (which is True effectively).

[lepture]

Then, how about as_pem, as_der? Should these methods return with private=False by default?

[geigerzaehler]

Yes, I’d suggest only returning public keys by default for all method. Otherwise it's quite easy to expose secret keys by accident