[FIX] web: pivot axis switch groupby field

In ee0caa246 we solved some issue when a field was used in a "group by"
filter, but was not considered "groupable" (which does not resolve
related field in their related model to see if they fit the bill).

This was fine, but this has to be also taken into account when these
group by are applied to columns, which is possible by switching x and y
axis.

opw-647401

addons/web/static/src/js/view_pivot.js

@@ -558,7 +558,7 @@ instance.web.PivotView = instance.web.View.extend({
             display_total = this.main_col.width > 1;
 
         var groupby_labels = _.map(this.main_col.groupbys, function (gb) {
-            return self.groupable_fields[gb.split(':')[0]].string;
+            return self.fields[gb.split(':')[0]].string;
         });
 
         for (i = 0; i < headers.length; i++) {