Zend\Db\Metadata\Source\AbstractSource Notice: Undefined index

I'm using my own Metadata source, but I'm pretty confident this occurs for the existing sources (Mysql, Postgresql, Sqlserver, ...).

```php
$metadata = new Zend\Db\Metadata\Metadata($adapter);
$constraints = $metadata->getConstraints('TABLE', 'SCHEMA');
foreach($constraints as $constraint) {
    $constraintKeys = $metadata->getConstraintKeys(
        $constraint->getName(),
        'TABLE', 'SCHEMA'
    );
}
```

Produces the following errors:

    Notice: Undefined index: constraint_references in library/Zend/Db/Metadata/Source/AbstractSource.php on line 393
    Notice: Undefined index: constraint_keys in library/Zend/Db/Metadata/Source/AbstractSource.php on line 400

Note that after this fixing this error I run into the next one:

     Fatal error: Class 'Zend\Db\Metadata\Object\ConstraintKeyObject' not found on line 398

Was that class just never committed?