x509-cert: incorrect bit tests for KeyUsage

[stormshield-guillaumed]

The recently added bit tests method on KeyUsage doesn't give the expected result.

Reproduction

let key_usage = KeyUsage(KeyUsages::KeyEncipherment.into());

println!("{}", key_usage.digital_signature());
println!("{}", key_usage.0.contains(KeyUsages::DigitalSignature));

Output

true
false

Expected

false
false

This is due to the usage of KeyUsages::X as u16 which is not the same as the flag value. I'm preparing a PR. Would you like to keep the same API or have a wrapper function fn contains(&self, rhs: impl Into<FlagSet<KeyUsages>>) or maybe both ?