title: FileHelpers DelimiterFieldBuilder issue
body: <p>I have this simple file.</p>
  
  <pre><code>first name 1,last name 1
  first name 2
  first name 3,last name 3
  first name 4,last name 4
  </code></pre>
  
  <ul>
  <li><p>Case 1: I read data from the file using this class:</p>
  
  <pre><code>    [DelimitedRecord(",")]
      public partial class Person
      {
          private string firstName;
  
          [FieldNullValue("default last name")]
          private string lastName;
  
          public string FirstName
          {
              get { return firstName; }
              set { firstName = value; }
          }
  
          public string LastName
          {
              get { return lastName; }
              set { lastName = value; }
          }
      }  
  </code></pre>
  
  <p>and this code</p>
  
  <pre><code>        ExcelStorage provider = new ExcelStorage(typeof(Person));
          provider.FileName = "data.csv";
          Person[] res = provider.ExtractRecords() as Person[];
  </code></pre></li>
  </ul>
  
  <p>I get the data correctly , the second element in the array has the last name = default last name.</p>
  
  <ul>
  <li><p>Case 2: I read the data using a dynamically build type:</p>
  
  <pre><code>        DelimitedClassBuilder cb = new DelimitedClassBuilder("Person2", ",");
          cb.AddField("firstName", typeof(string));
          cb.AddField("lastName", typeof(string));
          cb.LastField.FieldNullValue = "default last name";
          DelimitedFileEngine engine = new DelimitedFileEngine(cb.CreateRecordClass());
          DataTable dt = engine.ReadFileAsDT("data.csv"); 
  </code></pre></li>
  </ul>
  
  <p>I get the following exception : <code>Line: 2 Column: 0. Delimiter ',' not found after field 'firstName' (the record has less fields, the delimiter is wrong or the next field must be marked as optional).</code></p>
  
  <p>Let's change the file like this, adding separator on the second line</p>
  
  <pre><code>first name 1,last name 1
  first name 2,
  first name 3,last name 3
  first name 4,last name 4
  </code></pre>
  
  <ul>
  <li>Case 1 : Reading using the static typed class:
  I get the same result: there is a default last name</li>
  <li>Case 2: Reading using the dynamically created object:
  It doesn't brake anymore but there is no default value, there is just ""</li>
  </ul>