Component Element Separator is always null

The Component Element Separator (ISA16) appears to always be null. If I were to guess, because it is being parsed as an actual Component Element Separator. :-D

```c#
[Fact]
[Trait(Traits.Tag, "X12")]
public void X12_Component_Element_Separator_Should_Not_Be_Null() {
var grammar = EdiGrammar.NewX12();
var interchange = default(Models.PurchaseOrder_850);
using (var stream = Helpers.GetResourceStream("x12.850.edi")) {
interchange = new EdiSerializer().Deserialize(new StreamReader(stream), grammar);
}
Assert.NotNull(interchange.Component_Element_Separator);
}
```

P.S. Awesome library btw!

Start with the X12_Component_Element_Separator_Should_Not_Be_Null test, EdiGrammar.NewX12(), and EdiSerializer.Deserialize using x12.850.edi. Trace how the X12 interchange is parsed and how Component_Element_Separator is assigned. Done means the existing test passes with a non-null separator.

Written by the indexing model from the issue text.