String#chomp! should depend on $/ when argument not given.

MRI
---

* ruby 2.0.0dev (2012-12-07) [x86_64-linux]
* ruby 1.9.3p332 (2012-11-15) [x86_64-linux]
* ruby 1.8.7 (2012-10-12 patchlevel 371) [x86_64-linux]

```ruby
string = 'abc'
$/ = 'c'
string.chomp! #=> "ab"
string #=> "ab"
```

Rubinius
---------

* rubinius 2.0.0rc1 (1.9.3 6056ba1c 2012-11-02 JI) [x86_64-unknown-linux-gnu]
* rubinius 2.0.0rc1 (1.8.7 6056ba1c 2012-11-02 JI) [x86_64-unknown-linux-gnu]

```ruby
string = 'abc'
$/ = 'c'
string.chomp! #=> nil
string #=> "abc"
```