…red (apache#4409)

SshHelper.addIdentity() contained an early return after registering the
password identity:

    if (StringUtils.hasText(password)) {
        clientSession.addPasswordIdentity(password);
        return;          // <-- key never loaded
    }

This meant that whenever a Linux SSH monitor had both a password and a
private key configured, only the password was added to the MINA SSHD
client session. If the target server had PasswordAuthentication disabled
(public-key only), the connection always failed even though a valid
private key was present.

Fix: remove the early return so that both credentials are registered.
MINA SSHD will then attempt each method in order and succeed with
whichever the server accepts.

Changes:
- Removed the premature return in SshHelper.addIdentity() so that the
  private key is always loaded when privateKey is non-empty, regardless
  of whether a password is also set.
- Added regression test bothPasswordAndPrivateKeyAreRegisteredWhenBothConfigured
  in SshHelperTest that verifies both addPasswordIdentity and
  addPublicKeyIdentity are called on the client session when both
  credentials are supplied.

Fixes apache#4409