Add failing test for SQL literal equality with NULL

Hi, I've recently upgraded rails from `4.1.8` to `4.2.0.rc2` and I have small problem with ARel. This code:
```ruby
Arel.sql('my_literal').eq(nil)
```
generates `my_literal = NULL` instead of `my_literal IS NULL`.

I wrote one failing test for this. Is this behaviour is OK for new ARel?