title: CSVHelper C# DateTime column is throwing String is not a valid datetime
body: <p>I am using CSVHelper v4.0.30319.  I'm reading a CSV file that for a date has "//".  I was trying to override DefaultTypeConverter.ConvertFromString, but it is not being called. Below is my code.  How do I correctly handle this?</p>
  
  <p>How do I handle this?  </p>
  
  <pre><code>    public void Reader()
      {
          using (var sr = new StreamReader(@"test.csv"))
          {
              try
              {
                  var reader = new CsvReader(sr);
                  reader.Configuration.TypeConverterCache.AddConverter&lt;DateTime?&gt;(new DateTimeConverter());
  
                  IEnumerable&lt;DataRecord&gt; records = reader.GetRecords&lt;DataRecord&gt;();
  
                  foreach (DataRecord record in records.Take(5))
                  {
                      Debug.Print("{0} {1}, {2}, {3}", record.compid, record.standard, record.lastpmread,
                          record.pm_type001);
                  }
              }
              catch (Exception ex)
              {
                  Library.WriteErrorLog(ex);
              }
          }
      }
  </code></pre>