hash key downcase

from ruby facets:

class Hash

  def update_keys #:yield:
    if block_given?
      each{ |k,v| delete(k) ; store(yield(k), v) }
    else
      to_enum(:update_keys)
    end
  end

  def key_downcase!
    self.update_keys{|k| k.to_s.downcase}
  end

end