Union keys when passing array to except

Instead of intersecting keys - union them when doing
`set.except(filters)`.

When doing `Model.all.except(key: %w{some ids})`
it more sense to return (all - (some + ids))
instead of (all - (some & ids)).
Add test case for #except unions keys when passing an array.
Merge pull request #106 from frodsan/except_unions_keys

Except unions keys