Made oauth2 type field optional

Fixes this one https://github.com/Mashape/mashape-oauth/issues/10

![Dropbox Type Field](http://i.imgur.com/JhesXHR.png)

I didn't added a test for it, because by the spec this field should be required, but if you don't want it you can remove it by adding `type:null` to your settings (I'm going to push the fix for guardian as well)