Support magic properties

When i try to save an entity, the save() method return NULL.

I've seen that the properties of the Entity class are dynamically defined in entity.db.inc l.40:

// Set initial values.
    foreach ($values as $key => $value) {
      $this->$key = $value;
    }

But drupal_write_record() use the function property_exists() which check each property of the entity in includes/common.inc l.6684:

if (!property_exists($object, $field)) {
      // Skip fields that are not provided, default values are already known
      // by the database.
      continue;
    }

All the properties of my entity are skipped because they are dynamically defined.

On php.net you can read that note about property_exists:

"The property_exists() function cannot detect properties that are magically accessible using the __get magic method."