Change display of radio buttons Average column to more logical

When a radio-button or a yes/no question is set to "not required" and a number of respondents have skipped that question....
In the Questionnaire report View all responses page, on the choices lines, the Average column displays percentages of choices related to the total number of respondes. It would be more logical to display percentages related to total number of actual choices selected for those lines.
On the other hand, on the Total line, the current display of responses answered / total nb of responses can be retained, as it is meaningful.

The attached patch makes this new display logic possible.

Joseph