Fixed #21740 -- Stopped using mutable default arguments in test client

Thanks Denver Coneybeare for the report and initial patch, and
Atala for another patch.
Fixed #21784: Correctly decode stdin input for migration defaults
Fixed typo in docstring.
Merge pull request #2176 from numerodix/master

Fixed typo in docstring.
Fixed #18285 -- Documented that bulk_create doesn't work with M2M relationships.
Fixed #16785 -- Documented some of the design decisions of the cache framework.

Thanks jamesp for the draft patch.
Fixed #21785: Bad handling of both auto_now and _add on deconstruct