member_list is broken

Doing an API request with a valid group ID causes a server 500 error with the message:

    TypeError: <class 'ckan.model.user.User'> is not JSON serializable (from api controller)

Doing an API request with an invalid group ID causes a server 500 error with the message:

    ckan/ckan/logic/action/get.py', line 272 in member_list filter(model.Member.group_id == group.id).\
    AttributeError: 'NoneType' object has no attribute 'id'