Error in NA propogation in vectorized intersect

Thanks for a great package!

> intersect(interval("2014-01-15", "2014-01-16"), interval(NA,NA))
> [1] NA--NA
> 
> intersect(c(interval("2014-01-15", "2014-01-16"),interval("2014-01-15", "2014-01-16")), c(interval(NA,NA),interval(NA,NA)))
> Error in x@.Data[i] <- value@.Data : 
>   NAs are not allowed in subscripted assignments
