Fix next incomplete section button when first page of next subsection… (#786)

* Fix next incomplete section button when first page of next subsection is not routed to

* Improve spec context description

app/models/form.rb

@@ -79,7 +79,7 @@ class Form
     when :in_progress
       "#{next_subsection.id}/check_answers".dasherize
     when :not_started
-      first_question_in_subsection = next_subsection.pages.first.id
+      first_question_in_subsection = next_subsection.pages.find { |page| page.routed_to?(case_log, nil) }.id
       first_question_in_subsection.to_s.dasherize
     else
       "error"

spec/models/form_spec.rb

@@ -149,9 +149,9 @@ RSpec.describe Form, type: :model do
         expect(form.next_incomplete_section_redirect_path(subsection, case_log)).to eq("household-needs/check-answers")
       end
 
-      it "returns the first page of the next incomplete subsection (skipping completed subsections)" do
+      it "returns the first page of the next incomplete subsection (skipping completed subsections, and pages that are not routed to)" do
         answer_household_needs(case_log)
-        expect(form.next_incomplete_section_redirect_path(subsection, case_log)).to eq("accessible-select-too")
+        expect(form.next_incomplete_section_redirect_path(subsection, case_log)).to eq("property-postcode")
       end
 
       it "returns the declaration section for a completed case log" do