cor( x, y , method = "spearman" ) incorrect if any( is.na(c( x, y (PR#6448)

I think this would fix cor( ), but have not looked at the C-code for
additional gotcha's:

    if (method != "pearson") {
        Rank <- function(u) if (is.matrix(u)) 
            apply(u, 2, rank)
        else rank(u)
+       x.na <- is.na(x)
        x <- Rank(x)
+       is.na( x ) <- x.na
        if (!is.null(y)) {
+           y.na <- is.na(y)
            y <- Rank(y)
+           is.na(y) <- y.na
        }
    }