title: omniauth gowalla strategy error: comparison of String with Bignum failed
body: <p>as far as  I can get gowalla auth code response contains both <code>expires_at</code> and <code>expires_in</code> but
  expires_at is not valid date string so it throws error while comparing dates<br>
  any idea to hack omniauth or gowalla strategy would be very welcome!!!</p>
  
  <p>rails 3.0.9
  ruby 1.9.2</p>
  
  <pre><code>comparison of String with Bignum failed
  oauth2 (0.5.0) lib/oauth2/access_token.rb:72:in `&lt;'
  oauth2 (0.5.0) lib/oauth2/access_token.rb:72:in `expired?'
  oa-oauth (0.3.0) lib/omniauth/strategies/oauth2.rb:67:in `callback_phase'
  
  
  {
  "scope":"read",
  "expires_at":"Sun, 09 Oct 2011 12:47:37 -0000",
  "username":"altuure",
  "expires_in":1172767,
  "refresh_token":"XX",
  "access_token":"XX" 
  }
  </code></pre>