Forms refactoring - Registration Number form (#507)

From: https://eaflood.atlassian.net/browse/RUBY-693

Use delegate and strong parameters strategy in Registration Number form

app/controllers/waste_carriers_engine/registration_number_forms_controller.rb

@@ -9,5 +9,11 @@ def new
     def create
       super(RegistrationNumberForm, "registration_number_form")
     end
+
+    private
+
+    def transient_registration_attributes
+      params.require(:registration_number_form).permit(:company_no)
+    end
   end
 end

app/forms/waste_carriers_engine/registration_number_form.rb

@@ -2,31 +2,23 @@
 
 module WasteCarriersEngine
   class RegistrationNumberForm < BaseForm
-    attr_accessor :company_no, :business_type
+    delegate :company_no, :business_type, to: :transient_registration
 
     validates :company_no, "defra_ruby/validators/companies_house_number": true
 
-    def initialize(transient_registration)
-      super
-
-      self.company_no = transient_registration.company_no
-      # We only use this for the correct microcopy
-      self.business_type = transient_registration.business_type
-    end
-
     def submit(params)
       # Assign the params for validation and pass them to the BaseForm method for updating
       # If param isn't set, use a blank string instead to avoid errors with the validator
-      self.company_no = params[:company_no] || ""
-      self.company_no = process_company_no(company_no) if company_no.present?
-      attributes = { company_no: company_no }
+      params[:company_no] = process_company_no(params[:company_no])
 
-      super(attributes)
+      super
     end
 
     private
 
     def process_company_no(company_no)
+      return unless company_no.present?
+
       number = company_no.to_s
       # Should be 8 characters, so if it's not, add 0s to the start
       number = "0#{number}" while number.length < 8