Memo-ize FlowPresenter#start_node

`FlowPresenter#start_node` is called several times on the same instance within
a single request. This meant that we were unnecessarily creating several
instances of `StartNodePresenter`.

An instance of `SmartAnswer::ErbRenderer` is created for each of these and an
instance of `ActionView::Base` is created for each of those.

We can reuse the start node presenter by memo-izing it. It's safe to do this,
because a new `FlowPresenter` instance is created in every request within the
`SmartAnswersController#find_smart_answer` method.

Author: floehopper

app/presenters/flow_presenter.rb

@@ -95,7 +95,7 @@ def current_node
 
   def start_node
     node = SmartAnswer::Node.new(@flow, @flow.name.underscore.to_sym)
-    StartNodePresenter.new(node)
+    @start_node ||= StartNodePresenter.new(node)
   end
 
   def change_collapsed_question_link(question_number)

test/unit/flow_presenter_test.rb

@@ -81,4 +81,10 @@ class FlowPresenterTest < ActiveSupport::TestCase
     start_node = @flow_presenter.start_node
     assert_instance_of StartNodePresenter, start_node
   end
+
+  test '#start_node always returns same presenter for landing page node' do
+    start_node_1 = @flow_presenter.start_node
+    start_node_2 = @flow_presenter.start_node
+    assert_same start_node_1, start_node_2
+  end
 end