Fixes #2918 -- Clarified the db_prep_save logic for DateField and DateTimeField to prevent accidental conversion of non-datetime objects into strings, because SQLite doesn't appear to check for valid date format in a string used on an UPDATE of a datetime column.
Fixes #2918 -- Clarified the db_prep_save logic for DateField and DateTimeField to prevent accidental conversion of non-datetime objects into strings, because SQLite doesn't appear to check for valid date format in a string used on an UPDATE of a datetime column.


git-svn-id: 554f83ef17aa7291f84efa897c1acfc5d0035373@3960 bcc190cf-cafb-0310-a4f2-bffc1f526a37

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