Checks properly for existence of array element for quiz_log response_log

Current behaviour:

* Quiz Log's response_log field only records [null, null, null...]

Desired behaviour:

* Response_log should have a list of number correct in each attempt at the quiz.

Summary of changes:
* Add an "!" before checking whether an entry in the response_log exists.
Vindication!!! The world needs more exclamation marks!!!!

![image](https://cloud.githubusercontent.com/assets/618416/3805831/8e24d5aa-1c3e-11e4-9c84-f83eeec389a2.png)