* Moved dimension field update logic out of `ImageFileDescriptor.__set__` and into its own method on `ImageField`.
* New `ImageField.update_dimension_fields` method is attached to model instance's `post_init` signal so that:
* Dimension fields are set when defined before the ImageField.
* Dimension fields are set when the field is assigned in the model constructor (fixes#11196), but only if the dimension fields don't already have values, so we avoid updating the dimensions every time an object is loaded from the database (fixes#11084).
* Clear dimension fields when the ImageField is set to None, which also causes dimension fields to be cleared when `ImageFieldFile.delete()` is used.
* Added many more tests for ImageField that test edge cases we weren't testing before, and moved the ImageField tests out of `file_storage` and into their own module within `model_fields`.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@10858 bcc190cf-cafb-0310-a4f2-bffc1f526a37
