Add spec for single-item task lists

spec/lib/banzai/filter/task_list_filter_spec.rb

@@ -7,4 +7,10 @@ describe Banzai::Filter::TaskListFilter, lib: true do
     exp = act = %(<ul><li>Item</li></ul>)
     expect(filter(act).to_html).to eq exp
   end
+
+  it 'applies `task-list` to single-item task lists' do
+    act = filter('<ul><li>[ ] Task 1</li></ul>')
+
+    expect(act.to_html).to start_with '<ul class="task-list">'
+  end
 end