Enhance Rcpp::Nullable with explicitly cast as() export

[eddelbuettel]

In #1451 @cgiachalis suggested an Armadillo variant using a lambda which got us riffing a little. As shown in the (final ?) example, if we just enhance the existing Nullable::as() a little we can make this really right:

// [[Rcpp::export]]
arma::mat bis(Rcpp::Nullable<arma::mat> m = R_NilValue) {
    return (m.isNotNull() ? m.as() : arma::mat{0,0, arma::fill::none});
}

This utilizes a 'thirteen char' addition we should add.

modified   inst/include/Rcpp/Nullable.h
@@ -126,7 +126,7 @@ namespace Rcpp {
         /**
          * Returns m_sexp as a T
          */
-        inline T as() { return get(); }
+        inline T as() { return Rcpp::as<T>(get()); }
 
         /**
          * Return a clone of m_sexp as a T