Awaiting `HasSingleItem()` doesn't return the item anymore

Sorry for all the issues, and thanks for your work.

Previously, awaiting the method `HasSingleItem()` returned the element, instead of the enumerable.

Previously (IMHO the correct one):

```cs
Item singleItem = await Assert.That(list).IsNotNull().And.HasSingleItem();
```

Now:

```cs
IEnumerable<Item> items = await Assert.That(list).IsNotNull().And.HasSingleItem();
```