compileAttributes doesn't handle Dots, NamedDots

[kevinushey]

E.g. for this function:

// [[Rcpp::export]]
int countArgs(Dots dots){
    return dots.size() ;    
}

We should generate:

countArgs <- function(...) {
    .Call('RcppTest_countArgs', PACKAGE = 'RcppTest', environment())
}

instead of

countArgs <- function(dots) {
    .Call('RcppTest_countArgs', PACKAGE = 'RcppTest', dots)
}

So I guess we need to special case Dots and NamedDots.