Fixed #21499 -- Added a paragraph to the docs.
Fixed #21512 -- Added more complete information about Pillow and PIL in model_fields and model_forms tests.
Use str.isupper() to test if a string is uppercased.
Fixed #21355 -- try importing _imaging from PIL namespace first.
Fixed #21448 -- Fixed test client logout with cookie-based sessions

Thanks Gunnar Scherf for the report and the suggested patch.
Fixed a typo in the documentation
Replace use of dict.has_key with `key in dict`
Merge pull request #1985 from raphaa/21499

Fixed #21499 -- Migrations won't work if field signature changes