Refactor Investment csv download with filters test

There's no need to check again headers in this scenario, previous one
already does it.

Correctly naming variables, as well as using explicit expectations is a
good idea.

Last but not least, expectations where reversed but by luck or lack of
attention where passing.

spec/features/admin/budget_investments_spec.rb

@@ -991,22 +991,18 @@ feature 'Admin budget investments' do
     end
 
     scenario "Downloading CSV file with applied filter" do
-      investment1 = create(:budget_investment, :unfeasible, budget: budget,
-                                                            title: 'compatible')
-      investment2 = create(:budget_investment, :finished, budget: budget,
-                                                          title: 'finished')
+      unfeasible_investment = create(:budget_investment, :unfeasible, budget: budget,
+                                                                      title: 'Unfeasible one')
+      finished_investment = create(:budget_investment, :finished, budget: budget,
+                                                                  title: 'Finished Investment')
 
       visit admin_budget_budget_investments_path(budget)
       within('#filter-subnav') { click_link 'Valuation finished' }
 
       click_link "Download current selection"
 
-      header = page.response_headers['Content-Disposition']
-      expect(header).to match(/^attachment/)
-      expect(header).to match(/filename="budget_investments.csv"$/)
-
-      expect(page).to have_content investment2.title
-      expect(page).not_to have_content investment1.title
+      expect(page).to have_content('Finished Investment')
+      expect(page).not_to have_content('Unfeasible one')
     end
   end