Adds missing scope to investments

app/models/budget/investment.rb

@@ -42,6 +42,7 @@ class Budget
     scope :undecided,              -> { where(feasibility: "undecided") }
     scope :with_supports,          -> { where('cached_votes_up > 0') }
 
+    scope :by_heading,  -> (heading_id)  { where(heading_id: heading_id) }
     scope :by_budget,   -> (budget_id)   { where(budget_id: budget_id) }
     scope :by_admin,    -> (admin_id)    { where(administrator_id: admin_id) }
     scope :by_tag,      -> (tag_name)    { tagged_with(tag_name) }