case_insensitive_hash_writer.rb

require 'delegate'

class CaseInsensitiveHashWriter < SimpleDelegator
  def []=(key, value)
    return __getobj__[key] unless key.respond_to?(:downcase)
    matched_key = keys.detect do |k|
      k.respond_to?(:downcase) && k.downcase == key.downcase
    end
    if matched_key
      __getobj__[matched_key] = value
    else
      __getobj__[key] = value
    end
  end
end

h = {'fOO' => 'bar' }

cihc = CaseInsensitiveHashWriter.new(h)

cihc['Foo'] = 'baz'

h = cihc.to_h

puts h.class
puts h.inspect