Fix wrong evaluation of boolean kwargs

[falknes]

After spending too much time debugging why my extra entry in SOCIAL_AUTH_USER_FIELDS was None, if figured out that the issue lies in social.pipeline.create_user. If the extra field in SOCIAL_AUTH_USER_FIELDS is a boolean evaluated to False, and it´s provided in kwargs, it will return None instead of the correct False value.

>>> kwargs = {'field': False}
>>> details = {}
>>> results = kwargs.get('field') or details.get('field')
>>> print(results)
None
>>> results = kwargs.get('field', details.get('field'))
>>> print(results)
False

[omab]

Thanks!