Add headings_voted_by_user

This method was used only in Madrid’s fork, but it is now needed to
complete the backport for voting in multiple headings

There wasn’t a test in Madrid, so here goes one too. Even though, the
responsibility should probably be moved soon to the `Budget::Heading`.
For consistency with the related methods and tests it has been left in
the investment_spec

app/models/budget/investment.rb

@@ -239,6 +239,10 @@ class Budget
       headings_voted_by_user(user).count < group.max_votable_headings
     end
 
+    def headings_voted_by_user(user)
+      user.votes.for_budget_investments(budget.investments.where(group: group)).votables.map(&:heading_id).uniq
+    end
+
     def voted_in?(heading_ids, user)
       heading_ids.include? heading_voted_by_user?(user)
     end