Only columns for last row are displayed

I just updated to 7.x-1.x-dev that was released on Dec 2, 2015.

There is a regression in here that may take some time to explain.

I have a table group where the rows are defined by time and the columns field a field named room. The table group is only rendering columns that have values for the last row in the table.

This is the data I have

                  Room 1     Room 2    Room 3
10:00am    Biology      Comp. Sci  Math
11:00am    Lunch

However, we only get the columns present in the last row so end up with

                  Room 1   
10:00am    Biology 
11:00am    Lunch

This is happening because we have the following in tablegroup_plugin_style_tablegroup.inc

       foreach ($rows as $row_header => $row) {
        // Group by column:
        $columns = $this->render_grouping($row, $opt['col_grouping']);
        $cols = array_merge($cols, array_diff(array_keys($columns), $cols));
        $table['raw'][$row_header] = $columns;
      }

so $columns is the result of render_grouping for the last row.

Then we set up the table_headers based on the values in $columns.

I fixed this by keeping a separate array of all the columns found in all the rows and then using that to create table headers.