Possible broken assert in BaseFileKeyProvider.java

The `init` method in `BaseFileKeyProvider.java` looks like it has an invalid `assert` statement on line 62. Here's the current code:

```
@Override
public void init(String privateKey, String publicKey) {
assert privateKey != null;
assert publicKey == null;
resource = new PrivateKeyStringResource(privateKey);
}
```

The second line doesn't make sense since this method is called whenever we are using a `privateKey` and a `publicKey` on the `SSHClient` object. It seems like it should be this code instead:

```
@Override
public void init(String privateKey, String publicKey) {
assert privateKey != null;
assert publicKey != null;
resource = new PrivateKeyStringResource(privateKey);
}
```