Support nil as a meaningful param value

[emolayi]

To support a boolean field that is used as having a trinary value, I have had to override the filter_instructions_for method (see below). This is because a nil value is discarded.

# Override ActionSet filter_instructions_for method to modify filtering behavior.
  def filter_instructions_for(set)
    filter_params_hash = super

    # Allow `nil` values to be passed as meaningful filters on boolean fields
    filter_params.select { |_, v| v.include? 'nil' }.each do |k, v|
      modified_nil_value = nil if v.instance_of?(String) && v == 'nil'
      modified_nil_value = manipulate_trinary_boolean_array(v) if v.instance_of?(Array)
      filter_params_hash[k] = modified_nil_value if modified_nil_value
    end

    filter_params_hash
  end

  def manipulate_trinary_boolean_array(array)
    array.map do |b|
      next nil if b == 'nil'

      ActiveModel::Type::Boolean.new.cast(b)
    end
  end