return-value of mrb_run is invalid in top-level-scope

The return-value of mrb_run in top-level-scope is not the evaluated value at last from c209fa9514ea6296a798168481a769bd2225243d.

For example, the following mruby script by mrb_run does not return the value of `sum`.

```ruby
sum = 0
(1..10).each { |i| sum += i }
sum
```

In detail, when the above script is called by C, the return-value is not the value of `sum`.

```c
mrb_value result;
result = mrb_run(mrb, proc, mrb_top_self(mrb));
 
result = mrb_obj_as_string(mrb, result);
char *s = RSTRING_PTR(result);
printf("s:%s\n", s); // expected: 55, actual: 1..10
```

I don't know whether this is expected behavior. 
But I think better that the return-value of mrb_run in top-level-scope is the evaluated value at last.