Test case for `apply_sum()` wrong/missing

After taking a look at http://www.sqlalchemy.org/trac/wiki/05Migration and the module test/orm/deprecations.py referenced from therein, I found test_apply_avg() and test_apply_sum() to be identical (and probably suitable for the avg case), except for the first line of the latter adapted to the function name.

So the real test case for apply_avg() (or rather the replacement rendering it deprecated) is obviously missing and should be created.