title: Mapping for nullable property results in 0 but not Null
body: <p>I have the following mapping defined via FluentMapping</p>
  
  <pre><code>public class RuleMap : ClassMap&lt;Rule&gt;
  {
      public RuleMap()
      {
          Table("NEW");
  
          Id(x =&gt; x.Id, "Id").Not.Nullable();
          Map(x =&gt; x.SenderId, "SenderId").Nullable();
      }
  }
  </code></pre>
  
  <p>for the following class</p>
  
  <pre><code>public class Rule
  {
      public virtual int Id { get; set; }
  
      public virtual int? SenderId { get; set; }
  }
  </code></pre>
  
  <p>My Table in Database is defined as</p>
  
  <pre><code>USE [Test]
  GO
  
  /****** Object:  Table [dbo].[NEW]    Script Date: 23.04.2015 22:14:53 ******/
  SET ANSI_NULLS ON
  GO
  
  SET QUOTED_IDENTIFIER ON
  GO
  
  CREATE TABLE [dbo].[NEW](
      [Id] [int] NOT NULL,
      [SenderId] [int] NULL,
   CONSTRAINT [PK_NEW] PRIMARY KEY CLUSTERED 
  (
      [Id] ASC
  )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
  ) ON [PRIMARY]
  
  GO
  </code></pre>
  
  <p>and has a single Entry with</p>
  
  <pre><code>Id  | SenderId
   1  |  `NULL`
  </code></pre>
  
  <p>Now when I load all Rules via my SessionFactory</p>
  
  <pre><code>var rules = sessionFactory.QueryOvery&lt;Rule&gt;.List();
  </code></pre>
  
  <p>the list contains 1 Item (so far so good), but the property <code>SenderId</code> has the value <strong>0</strong> and <strong>not NULL</strong> what I have expected.</p>
  
  <p>So what am I doing wrong here?</p>
  
  <p>Using NHibernate 3.1 and FluentNHibernate 1.2.</p>