title: remove collection with delete-orphan not work with null assignment? :(
body: <p>I am having problems removing another entity through cascade delete-orphan.
  It works when I clear the associated set collection, but not when I make the set collection null.
  Let me explain in detail.
  The config snippet:</p>
  
  <pre><code>&lt;class name="com.sample.CategoriesDefault" table="cats"&gt;
    &lt;id name="id" column="id" type="string" length="40" access="property"&gt;
       &lt;generator class="assigned" /&gt;
    &lt;/id&gt;
    &lt;version name="version" column="objVrs"  unsaved-value="negative"/&gt;
  
   &lt;set name="bla" lazy="false" cascade="all-delete-orphan" inverse="true"&gt;
        &lt;key column="idCats" not-null="true"/&gt;
         &lt;one-to-many class="com.sample.BLA"/&gt;
   &lt;/set&gt;
  
  &lt;class name="com.sample.BLA" table="blaTEST"&gt;
     &lt;id name="id" column="id" type="string" length="40" access="property"&gt;
       &lt;generator class="assigned" /&gt;
     &lt;/id&gt;
     &lt;version name="version" column="objVrs"  unsaved-value="negative"/&gt;
     &lt;property name="bla" type="string" column="bla"/&gt;
     &lt;many-to-one name="parent" class="com.sample.CategoriesDefault" column="idCats" not-null="true"/&gt;
  &lt;/class&gt;
  </code></pre>
  
  <p>My sample code:</p>
  
  <pre><code>Categories cats = new CategoriesDefault();
  final Set&lt;BLA&gt; col = new HashSet&lt;BLA&gt;();
  col.add(new BLA(cats));
  cats.setBla(col);
  cats.saveOrupdate(); // will update/insert it in the db.
  </code></pre>
  
  <p>The following works correctly, that is: all the collection items are all moved from the db.</p>
  
  <pre><code>cats.getBla().clear();
  cats.saveOrUpdate();
  </code></pre>
  
  <p>I think this works as the PersistSet of Hibernate is marked as dirty when calling this method.</p>
  
  <p>The following however doesn't work the same as above like I would want/expect. </p>
  
  <pre><code>cats.setBla(null);
  cats.saveOrUpdate();
  </code></pre>
  
  <p>If I reload the cats item from the db, it still contains the BLA items and no delete statement is generated by Hibernate :(..
  Why not??... or is this a bug ? I am using 3.6.0.Final.</p>