Fix edge case

The user was able to vote as many investments as wanted in the first
heading voted. However in the second heading voted, only one investment
could be voted

This was due to the previous implementation, where you could only vote
in one heading. Note the `first` call in method
`heading_voted_by_user?(user)`

This commits simplifies the logic and allows voting for any investment
in any heading that the user has previously voted in

app/models/budget/investment.rb

@@ -231,7 +231,7 @@ class Budget
     end
 
     def valid_heading?(user)
-      voted_in?([heading.id], user) ||
+      voted_in?(heading, user) ||
       can_vote_in_another_heading?(user)
     end
 
@@ -243,13 +243,8 @@ class Budget
       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
-
-    def heading_voted_by_user?(user)
-      user.votes.for_budget_investments(budget.investments.where(group: group))
-          .votables.map(&:heading_id).first
+    def voted_in?(heading, user)
+      headings_voted_by_user(user).include?(heading.id)
     end
 
     def ballotable_by?(user)