Use class_eval to override link_to_cart in Solidus Demo

Goal
----

As a Solidus Demo contributor

I would like to use `class_eval` to override link_to_cart

So that we can ensure that the `Spree::BaseHelper` engine module is
reopened by Solidus Demo, which according to
https://guides.rubyonrails.org/engines.html#improving-engine-functionality,
is necessary for overriding the module.

Basis
-----

From
https://guides.rubyonrails.org/engines.html#improving-engine-functionality:

> It is very important that the override reopens the class or module.
Using the class or module keywords would define them if they were not
already in memory, which would be incorrect because the definition lives
in the engine. Using `class_eval` as shown above ensures you are
reopening.

app/monkey_patches/spree/base_helper_monkey_patch.rb

@@ -1,18 +1,16 @@
-module Spree
-  module BaseHelper
-    def link_to_cart(text = nil)
-      text = text ? h(text) : t('spree.cart')
-      css_class = nil
+Spree::BaseHelper.class_eval do
+  def link_to_cart(text = nil)
+    text = text ? h(text) : t('spree.cart')
+    css_class = nil
 
-      if current_order.nil? || current_order.item_count.zero?
-        text = ""
-        css_class = 'empty'
-      else
-        text = "<div class='link-text'>#{current_order.item_count}</div>"\
-              #"<span class='amount'>#{current_order.display_total.to_html}</span>"
-        css_class = 'full'
-      end
-      link_to text.html_safe, spree.cart_path, class: "cart-info #{css_class}"
+    if current_order.nil? || current_order.item_count.zero?
+      text = ""
+      css_class = 'empty'
+    else
+      text = "<div class='link-text'>#{current_order.item_count}</div>"\
+            #"<span class='amount'>#{current_order.display_total.to_html}</span>"
+      css_class = 'full'
     end
+    link_to text.html_safe, spree.cart_path, class: "cart-info #{css_class}"
   end
 end