Using null filter as part of comma delineated filter list does not typecast 'null' to nil

[smai-f]

If filtering by nil is enabled on your resource and you use a filter such as filter[name]=null,Lucy, it would be nice if this filter returned all values that were either nil or Lucy. Instead, the filter value ends up here as ['null', 'Lucy'] -- 'null' is a string and does not get typecast to nil.

Only verified with resource backed by ActiveRecord.