NH-2241 - IStatelessSession is accidentally 2LC enabled in some cases

[igitur]

NH-2241 - When inserting an entity that references an entity with assigned id causes exception if that entity as a cache property.

Example:

<class name="Document" >
    <cache region="ShortTerm" usage="read-write" />
    <id name="Name" >
        <generator class="assigned"/>
    </id>
    <property name="Text"/>
</class>
<class name="Paper">
    <id name="Id">
        <generator class="native"/>
    </id>
    <property name="Color"/>
    <many-to-one name="Document" column="DocId" cascade="none" />
</class>

When inserting a Paper entity using BatchInsert in the stateless session throws NotSupported exception from the Timestamp property on the session (ISessionImplementor).

This happens in the end of the IsTransient method (of the AbstractEntityPersister):

// check to see if it is in the second-level cache
if (HasCache)
{
    CacheKey ck = new CacheKey(id, IdentifierType, RootEntityName, session.EntityMode, session.Factory);
    if (Cache.Get(ck, session.Timestamp) != null)
        return false;
}

Also logged at https://nhibernate.jira.com/browse/NH-2241

This PR adds a unit test, but unfortunately I couldn't find a solution for this bug.

[igitur]

Thank you for the quick fix. Much appreciated.