ModelViews don't correctly validate Resources whose models have unique fields.

[fzunino]

With this patch ModelViews can have Resources whose models have unique fields.

ReadModelMixin and UpdateModelMixin store model instance as a property. This allows ModelResource to bind the ModelForm using the model instance making the form validate the input data against the model instance and not a brand new instance. When the latter happened and the model used unique fields, the form validation failed whenever a PUT was maintaining the previous value of the unique field.

Example to reproduce:

class Movie(models.Model):
    name = models.CharField(max_length=200)
    imdb_id = models.CharField(max_length=32, blank=True, unique=True, null=True)

class MovieResource(ModelResource):
    model = Movie
    fields = ('name', 'imdb_id', 'url')
    ordering = ('name',)

urlpatterns = patterns('',
    url(r'^movies/$', ListOrCreateModelView.as_view(resource=MovieResource), name='movies'),
    url(r'^movies/(?P<id>[0-9]+)/$', InstanceModelView.as_view(resource=MovieResource)),
)

$ curl -X POST --data 'name=movie&imdb_id=1' http://localhost:8000/movies  
201 Created

$ curl -X PUT --data 'name=movie&imdb_id=1' http://localhost:8000/movies/1  
400 Bad Request