Fixed authentication using Dropbox

Fixed authentication using Dropbox as reported in https://github.com/gae-init/gae-init-auth/issues/34 based on example code provided at https://github.com/lepture/flask-oauthlib/blob/master/example/dropbox.py
Seems the pop with default is used to avoid key errors... http://stackoverflow.com/a/20115928/2315612