Use a virtual attribute to get valuation tags

It was strange to set the valuation tags using `valuation_tag_list=` but
then accessing the valuation tags using `tag_list_on(:valuation)`.

app/models/budget/investment.rb

@@ -349,6 +349,10 @@ class Budget
       self.valuator_groups.collect(&:name).compact.join(', ').presence
     end
 
+    def valuation_tag_list
+      tag_list_on(:valuation)
+    end
+
     def valuation_tag_list=(tags)
       set_tag_list_on(:valuation, tags)
     end

app/views/admin/budget_investments/edit.html.erb

@@ -57,7 +57,7 @@
         <% end %>
       </div>
       <%= f.text_field :valuation_tag_list,
-                        value: @investment.tag_list_on(:valuation).sort.join(','),
+                        value: @investment.valuation_tag_list.sort.join(','),
                         label: false,
                         placeholder: t("admin.budget_investments.edit.tags_placeholder"),
                         class: 'js-tag-list' %>

spec/models/budget/investment_spec.rb

@@ -642,6 +642,14 @@ describe Budget::Investment do
         results = described_class.search("Latin")
         expect(results.first).to eq(investment)
       end
+
+      it "gets and sets valuation tags through virtual attributes" do
+        investment = create(:budget_investment)
+
+        investment.valuation_tag_list = %w[Code Test Refactor]
+
+        expect(investment.valuation_tag_list).to match_array(%w[Code Test Refactor])
+      end
     end
 
   end