New Post: CollectionPropertyAssertions

public static IPropertyAssertions<T> ShouldHave<T>(this T subject)
{
    return new PropertyAssertions<T>(subject);
}

public static IPropertyAssertions<T> ShouldHave<T>(this IEnumerable<T> subject)
{
    return new CollectionPropertyAssertions<T>(subject);
}

var collection1 = new List<Customer>(); //first overload is chosen
List<Customer> collection2 = new List<Customer>(); //first overload is chosen
IEnumerable<Customer> collection3 = new List<Customer>(); //second overload is chosen