Issue Description:

Given the following snippet:

var a: [Int?] = [1,2,nil]
a = a.flatMap { (item: Int?) -> Int? in
    return item
}
print(a)

I expect the return value to be [1,2,nil], but it returns [1,2] instead.

My rationale is that the matching function is...

extension SequenceType {
    @warn_unused_result
    public func flatMap<T>(@noescape transform: (Self.Generator.Element) throws -> T?) rethrows -> [T]
}

... so the return type of flatMap must be [Int?] to match the type of a, that implies T is Int?. That means the return type of transform must be Int??. Our closure returning Int? is properly accepted as it is covariant [0]. However, my expectation is that if our closure returns .None it would be treated as .Some(.None) ultimately by the transform, since .None is a valid Int? for wrapping in Int??.

[0]: Indeed, this code does not compile in 2.0, before closure covariance support was added.