headers.rb

require 'set'

class HTTP::Headers
  def initialize
    @pile = []
    @keys = Set.new
  end

  def set(name, value)
    delete(name)
    add(name, value)
  end

  alias_method :[]=, :set

  def delete(name)
    name = normalize_header name
    @keys.delete name
    @pile.delete_if { |k, _|, k == name }
  end

  def add(name, value)
    name = normalize_header name
    @keys << name
    Array(value).each { |v| @pile << [name, v] }
  end

  def get(name)
    name = normalize_header name
    @pile.select { |k, _| k == name }.map { |_, v| v }
  end

  def [](name)
    values = get(name)
    1 == values.count ? values.first : values
  end

  def to_h
    Hash[@keys.map { |k| [k, self[k]] }]
  end

  def keys
    @keys.to_a
  end

  private

  def normalize_header(name)
    # ...
  end
end