Fixnum#to_s seems to generate strings immune to #[]=

Test code

``` ruby
x = 100.to_s
x[0] = '2'
p x
```

MRI result

``` ruby
"200"
```

JRuby result

``` ruby
"100"
```

Tested against 
`ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin12.5.0]` and `jruby 1.7.2 (1.9.3p327) 2013-01-04 302c706 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_12-ea-b08 +indy [darwin-x86_64]`

see https://gist.github.com/dennyabraham/9400684
