Added a test to ensure bad assignation to M2M doesn't clear data

Refs #14394.
Complemented pickle test for ValuesQuerySet with related field

Refs #14515.
Fixed #22461 -- Added if-unmodified-since support to the condition decorator.
Fixed #10414 -- Made select_related() fail on invalid field names.
Fixed #24041 -- Documented effect of changing a model instance's primary key.
Fixed #24049 -- Removed obsolete Java notes for Mac OS.
Removed shutil.copystat copied from stdlib (added to support an old Python?).
Fixed typo in docs/intro/tutorial01.txt.
Fixed #23866 -- Harmonized refs to Django documentation from code
Updated link to CSRF docs

Refs #23866.
Fixed #24045 -- Removed useless mark_safe() call in trans_null.py