nil param value that expects a hash should return :key => nil

When a nil param value arrives in place of an expected hash, return :key => nil, not simply nil.

Prior:

    (rdb:1) params
    {"book"=>nil}
    (rdb:1) params.permit(book: { genre: :type })
    nil

Now:

    (rdb:1) params
    {"book"=>nil}
    (rdb:1) params.permit(book: { genre: :type })
    {"book"=>nil}