session_hash.rb

class SessionHash < Hash

  #...

  def [](key)
    load_for_read!
    @data[key.to_s]
  end

  def []=(key, value)
    load_for_write!
    @data[key.to_s] = value
  end
end

sinatra.rb

  def indifferent_params(object)
    case object
    when Hash
      new_hash = indifferent_hash
      object.each { |key, value| new_hash[key] = indifferent_params(value) }
      new_hash
    when Array
      object.map { |item| indifferent_params(item) }
    else
      object
    end
  end

  # Creates a Hash with indifferent access.
  def indifferent_hash
    Hash.new {|hash,key| hash[key.to_s] if Symbol === key }
  end

The first way is the way Rack does this:
https://github.com/rack/rack/blob/master/lib/rack/session/abstract/id.rb#L59

The second way is how Sinatra does it for params:
https://github.com/sinatra/sinatra/blob/master/lib/sinatra/base.rb#L996