Please remove IEnumerable requirement for Collection Initializers

[TonyValenti]

Hi C# Team,
Would you please remove the requirement that a class implement IEnumerable in order to utilize collection initializers?

It would be really nice to be able to do things like:

var H = new HashCode()
            {
                Start_At,
                End_At,
            };

[svick]

This is a duplicate of #455.