DatePreposition is generating the wrong SQL to find NULL values

[boblail]

I think this is a bug in Arel:

people[:birthday].not_eq(nil) # => "people.birthday IS NOT NULL"
Arel.sql("values.value::date").not_eq(nil) # => "values.value != NULL"

The latter is invalid syntax.

This PR has a failing test, but not a fix.