Does not handle polymorphic types

[mhluska]

For example:

{
  "data" => {
    "attributes" => { ... }, 
    "relationships" => {
      "payable" => {
        "data" => {
          "type" => "orders", 
          "id" => "535"
        }
      }
    }, 
    "type" => "payments"
  }
}

This would give nil for Payment.new(restify_param(:payment).require(:payment).permit(:payable_type, :payable_id).payable