Trying to use Webmock's to_rack functionality and Rails 4 and getting huge errors with the session object? This monkeypatch provides the missing 'rack.session.options' key to the request environment.

gistfile1.rb

require 'webmock/minitest'

module WebMock
  class RackResponse < WebMock::Response

    def build_rack_env(request)
      uri = request.uri
      headers = request.headers || {}
      body = request.body || ''

      env = {
        # CGI variables specified by Rack
        'REQUEST_METHOD' => request.method.to_s.upcase,
        'CONTENT_TYPE'   => headers.delete('Content-Type'),
        'CONTENT_LENGTH' => body.size,
        'PATH_INFO'      => uri.path,
        'QUERY_STRING'   => uri.query || '',
        'SERVER_NAME'    => uri.host,
        'SERVER_PORT'    => uri.port,
        'SCRIPT_NAME'    => ""
      }

      env['HTTP_AUTHORIZATION'] = 'Basic ' + [uri.userinfo].pack('m').delete("\r\n") if uri.userinfo

      # Rack-specific variables
      env['rack.input']      = StringIO.new(body)
      env['rack.errors']     = $stderr
      env['rack.version']    = Rack::VERSION
      env['rack.url_scheme'] = uri.scheme
      env['rack.run_once']   = true
      env['rack.session']    = session
      env['rack.session.options'] = session_options # include session options to keep rails 4 from asploding

      headers.each do |k, v|
        env["HTTP_#{k.tr('-','_').upcase}"] = v
      end

      env
    end

    def session_options
      @session_options ||= {}
    end

  end
end