Filtering JSONB Array Throws Error

Attempting to filter a jsonb array:

`filter_on :tags, type: :jsonb`

Throws an error:

```
undefined local variable or method `val' for #<Sift::WhereHandler:0xb0fe4148 @param=#<Sift::Parameter:0xb131ed98 @param=:tags, @type=:jsonb, @internal_name=:tags>>
```

It looks like this is a simple typo in `Sift::WhereHandler`. I will submit a pull request with a fix.