Fix Failing functional test

Fix failing functional test due to issue #813.

The issue was when post_add was called in the test case, it created a deduction of 0.  Under normal situation (non test), when a submission is collected, the deduction previous created will be removed and recalculated.  However, in the test case I wrote, the deduction was not removed prior to adding a new deduction on top.  Therefore, when running the command self.grace_period_deduction_single in grouping.rb, there was an extra deduction of 0 that should not have been there and most of the time when the test case ran, it returned it instead of the expected correct deduction.