fix RObject#sum,prod (#169)

Author: masa16

ext/numo/narray/gen/spec.rb

@@ -274,7 +274,7 @@
   cond_unary "isfinite"
 end
 
-if is_int
+if is_int && !is_object
   if is_unsigned
     accum "sum","u_int64_t","numo_cUInt64"
     accum "prod","u_int64_t","numo_cUInt64"