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This allows DataList::create(SiteTree) as equivalent to Object::create(DataList, SiteTree), without
having to have a create() function on DataList.
Required for E_STRICT compliance, as child classes cant override create() if they change the arguments.
DBField::create() is also renamed to DBField::create_field(), as this does not just call the constructor, which all other cases of create() do.
Conflicts:
tests/model/DateTest.php
tests/model/DatetimeTest.php