Overwrite config params for throw-away clients in the service builder

```php
$builder = new ServiceBuilder(array(
    'client' => array(
        'class' => 'My\Cool\Client',
        'params' => array(
            'base_url' => 'http://www.my-url.com/',
            'foo'      => 'bar',
        )
    )
));

// Normal client with normal config
$client1 = $builder->get('client');

// Throw-away client with normal config
$client2 = $builder->get('client', true);

// Throw-away client with modified config
$client3 = $builder->get('client', array('foo' => 'baz'));
```