Recursive hash merge in Ruby

gistfile1.rb

def merge_recursively(a, b)
    result = Hash.new
    a.each do |key, a_item|
      if b.key?(key)
        b_item = b[key]
        if b_item.is_a? Hash and a_item.is_a? Hash
          result[key] = merge_recursively(a_item, b_item)
        else
          result[key] = b_item.nil? ? a_item : b_item
        end
      else
        result[key] = a_item
      end
    end

    b.each do |key, b_item|
      unless a.key?(key)
        result[key] = b_item
      end
    end
    result
  end