Make set_original_heading_id task idempotent

This way there'll be no side effects if accidentally executed on data
already having the `original_heading_id`.

spec/lib/tasks/budgets_spec.rb

@@ -3,6 +3,7 @@ require "rails_helper"
 describe Budget do
 
   let(:run_rake_task) do
+    Rake::Task["budgets:set_original_heading_id"].reenable
     Rake.application.invoke_task("budgets:set_original_heading_id")
   end
 
@@ -19,4 +20,17 @@ describe Budget do
     expect(investment.original_heading_id).to equal(heading.id)
   end
 
+  it "does not overwrite original_heading_id when already present" do
+    original_heading = create(:budget_heading)
+    new_heading = create(:budget_heading)
+    investment = create(:budget_investment, heading: original_heading)
+    investment.update(heading: new_heading)
+
+    expect(investment.original_heading_id).to eq original_heading.id
+
+    run_rake_task
+    investment.reload
+
+    expect(investment.original_heading_id).to eq original_heading.id
+  end
 end