really fix https://github.com/tomchristie/django-rest-framework/issues/180

The bug was caused by unpredictable set ordering with duplicated fields:

    default = self.get_default_fields(obj)
    include = self.include or ()
    exclude = self.exclude or ()
    fields = set(default + list(include)) - set(exclude)

If include is specified as [('fieldname', Serializer)], it will not override
'fieldname' element in list returned by get_default_fields(). Resulting set
will contain both 'fieldname' and ('fieldname', Serializer) and will be
processed in unpredictable order.