code: public SslContextBuilder keyManager(PrivateKey key, X509Certificate... keyCertChain) {
          return keyManager(key, null, keyCertChain);
      }
docs: Identifying certificate for this host. {@code keyCertChain} and {@code key} may
  be {@code null} for client contexts, which disables mutual authentication.
  
  @param key a PKCS#8 private key
  @param keyCertChain an X.509 certificate chain