Add test for atomic update with F of a field with a different db_column

[bikeshedder]

Atomic updates with F do not work when the field uses a different db_column.

For now I have only prepared a test, which shows the wrong behavior.

[jonashaag]

Looks good to me. Could you also come up with a fix?

Update spec generation is handled in compiler.py:SQLUpdateCompiler.update (around lines 420 - 450).

The problem is that in line 444, lhs.name is used instead of field.column. It's save to just replace it provided that the code along the lines 441-443 is never executed (which needs to be confirmed for instance by using a coverage tool -- runtests.py has an option to do that). I'm not sure why 441-443 exist in the first place, the code is sort of just c&p-ed from Django's SQL backend.