.or with interval and on same field not working

https://github.com/mongoid/mongoid/issues/3110 reported this issue.

Which basically this query:

``` ruby
query.or({:first.gt => 1, :first.lte => 3}, {:first.gt => 20, :first.lte => 30 })
```

is returning this selector

``` ruby
[{"first"=>{"$lte"=>3}}, {"first"=>{"$lte"=>30}}]
```

Which is incorrect.
