[IronPython] Default-valued arguments to C# functions

[Dino Viehland]

Jeff wrote:
> Interestingly, using DefaultValue worked for anything that wasn't an
> object - `[Optional][DefaultValue(-1)]int index` worked just fine.

I think you should have been getting a value of 0 when the parameter
wasn't being provided in this case.  When something is optional but
not typed to object we need to pass default(typeof(T)) instead of passing
Missing.Value.