Fix cascade remove for associations.

Option casade={"remove"}  does not working on second and deeper level of cascade.
Because in association we are have proxy object which not loaded. 
And call of \ReflectionProperty::getValue on this object returns defalt value, not real related documents.
``` php
$relatedDocuments = $class->reflFields[$mapping['fieldName']]->getValue($document); 
```
Cascade delete continues without affecting nested objects

This commit fixed this bug.

Mapping Example:

My user class:
``` php
 // User.php
use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;

/**
* @MongoDB\Document
*/
class User 
{
    /**
     * @var string
     *
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @var string
     *
     * @MongoDB\String
     */
    protected $username;

    /**
     *  @MongoDB\ReferenceMany(targetDocument="House", mappedBy="user", cascade="remove")
     */
    protected $houses;
}
```

My house class:
``` php
 // House.php
use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;

/**
* @MongoDB\Document
*/
class House 
{
    /**
     * @var string
     *
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @var User
     *
     * @MongoDB\ReferenceOne(targetDocument="User", inversedBy="houese")
     */
    protected $user;

    /**
     *  @var Photo[]
     *
     *  @MongoDB\ReferenceMany(targetDocument="House", cascade="remove")
     */
    protected $photos;
}
```

My photo class:
``` php
 // Photo.php
use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;

/**
* @MongoDB\Document
*/
class Photo 
{
    /**
     * @var string
     *
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @var string
     *
     * @MongoDB\String
     */
    protected $filename;
}
```

With this mapping when i execute 
``` php
$documentManager->remove($someUserDocument);
```
All houses associated with `` $someUserDocument `` will be removed from mongo, but all photos associated with this houses will not be removed