String#upto should call block when increase digit for x19

MRI(ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-linux])

```ruby
'9'.upto('10').to_a #=> ["9", "10"]
```

Rubinius(rubinius 2.0.0.rc1 (1.9.3 74955280 yyyy-mm-dd JI) [x86_64-unknown-linux-gnu])

```ruby
 '9'.upto('10').to_a #=> []
```