obvious fix for #209. not so obvious to deal with the consequences

Author: romainfrancois

inst/include/Rcpp/vector/proxy.h

@@ -240,7 +240,7 @@ namespace traits {
 	};
 	template <int RTYPE>
 	struct r_vector_const_iterator {
-		typedef typename storage_type<RTYPE>::type* type ;
+		typedef const typename storage_type<RTYPE>::type* type ;
 	};
 
 	template <int RTYPE> struct proxy_based_iterator{