After being sent to https://twitter.com/oauth/authorize, if the user then
clicks the "Sign Out" link, she is redirected back to /oauth/authorize, but
the value of oauth_access_type gets lost.

* Steps to reproduce

1. Log in to twitter.com.

2. Using the test page at

      http://dairiki.test.discnw.org/twitter/oauth/test.html

   attempt to authorize for read-only access.

3. Once at /oauth/authorize, note that the proper access type (read-only)
   is being requested.

4. Click the "Sign Out" link.   After being signed out, you
   will be redirected back to /oauth/authorize.   
   Here's the bug: the oauth_access_type parameter is gone ---
   now the default access type for the app (read/write) is being
   requested.