Calculator 'None' is the default one for both shipping and payment method

+ update specs as well

Update shipping_method.rb

app/models/spree/payment_method.rb

@@ -113,7 +113,7 @@ module Spree
     end
 
     def init
-      self.calculator ||= ::Calculator::FlatRate.new(preferred_amount: 0)
+      self.calculator ||= ::Calculator::None.new
     end
 
     def has_distributor?(distributor)

app/models/spree/shipping_method.rb

@@ -32,6 +32,8 @@ module Spree
     validate :at_least_one_shipping_category
     validates :display_on, inclusion: { in: DISPLAY_ON_OPTIONS.values }, allow_nil: true
 
+    after_initialize :init
+
     after_save :touch_distributors
 
     scope :managed_by, lambda { |user|
@@ -110,6 +112,10 @@ module Spree
       where(display_on: [nil, ""])
     end
 
+    def init
+      self.calculator ||= ::Calculator::None.new
+    end
+
     private
 
     def no_active_or_upcoming_order_cycle_distributors_with_only_one_shipping_method?

spec/system/admin/payment_method_spec.rb

@@ -247,10 +247,13 @@ describe '
   end
 
   describe "Setting transaction fees", js: true do
-    let(:calculator) { build(:calculator) }
-    let!(:payment_method) { create(:payment_method, calculator: calculator) }
+    let!(:payment_method) { create(:payment_method) }
     before { login_as_admin_and_visit spree.edit_admin_payment_method_path payment_method }
 
+    it "set by default 'None' as calculator" do
+      expect(page).to have_select "calc_type", selected: "None"
+    end
+
     it "handle the 'None' calculator" do
       select2_select "None", from: 'calc_type'
       click_button 'Update'
@@ -273,9 +276,11 @@ describe '
     end
 
     context "using Flat Rate (per order) calculator" do
-      # flat rate per order is the default calculator; no need select it and update page
+      before { select2_select "Flat Rate (per order)", from: 'calc_type' }
 
       it "inserts values which persist" do
+        expect(page).to have_content("you must save first before")
+        click_button 'Update'
         fill_in "Amount", with: 2.2
         click_button 'Update'
         expect(page).to have_content("Payment Method has been successfully updated!")