title: CsvHelper ConvertUsing not changing output
body: <p>I'm trying to use the <code>ConvertUsing</code> method of the <a href="https://github.com/JoshClose/CsvHelper">CsvHelper</a> library (v 2.4.0).</p>
  
  <p>I've read the documentation about <a href="http://joshclose.github.io/CsvHelper/#mapping-convert-using">ConvertUsing</a> but can't get it to work.</p>
  
  <p>I'm using a simple class:</p>
  
  <pre><code>public class Test
  {
      public long Id { get; set; }
      public string Title { get; set; }
  }
  </code></pre>
  
  <p>With this <code>ClassMap</code>:</p>
  
  <pre><code>public class TestClassMap : CsvClassMap&lt;Test&gt;
  {
      public override void CreateMap()
      {
          Map(m =&gt; m.Id).Name("id").ConvertUsing(row =&gt; 11111);
          Map(m =&gt; m.Title).Name("title").ConvertUsing(row =&gt; row.GetField("title") + " 123");
      }
  }
  </code></pre>
  
  <p>My code which uses these creates an instance of the class and then writes it to CSV:</p>
  
  <pre><code>var test = new Test() { Id = 99, Title = "Test title" };
  
  using (var streamWriter = new StreamWriter("test.csv"))
  {
      var csv = new CsvWriter(streamWriter);
      csv.Configuration.RegisterClassMap&lt;TestClassMap&gt;();
      csv.WriteRecord(test);
  }
  </code></pre>
  
  <p>However the output file <code>test.csv</code> is always the following format:</p>
  
  <pre><code>id,title
  99,Test title
  </code></pre>
  
  <p>The output I'm looking for is:</p>
  
  <pre><code>id,title
  11111,Test title 123
  </code></pre>
  
  <p>And the <code>ConvertUsing</code> is being ignored. I've tried only converting the <code>Id</code>, and only the <code>Title</code>, but this doesn't work either.</p>
  
  <p>Any ideas where I'm going wrong?</p>