Nil HTTP user agent breaks route constraints on user_agent

Using the following code in routing (which is recommended on many sites) raises an exception deep inside rack if no user agent is provided by the client:

  constraints :user_agent => /MSIE 6/i do
    get 'admin' => redirect('/unsupported_browser')
  end

Not sure where the correct place to fix this is, but attached is one option in two diffs:
1) Failing test against rails master
2) Fix for rack-mount