Added a spec to test wiki with json request

app/controllers/concerns/wiki_actions.rb

@@ -8,6 +8,8 @@ module WikiActions
   extend ActiveSupport::Concern
 
   included do
+    before_action { respond_to :html }
+
     before_action :authorize_read_wiki!
     before_action :authorize_create_wiki!, only: [:edit, :create]
     before_action :authorize_admin_wiki!, only: :destroy

changelogs/unreleased/209910-wiki-respond-not_acceptable.yml

+---
+title: Wiki controller should not crash with non-html format
+merge_request: 37466
+author:
+type: fixed

spec/support/shared_examples/controllers/wiki_actions_shared_examples.rb

@@ -61,6 +61,14 @@ RSpec.shared_examples 'wiki controller actions' do
       expect(assigns(:sidebar_wiki_entries)).to be_nil
       expect(assigns(:sidebar_limited)).to be_nil
     end
+
+    context 'when the request is of non-html format' do
+      it 'returns a 404 error' do
+        get :pages, params: routing_params.merge(format: 'json')
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
   end
 
   describe 'GET #history' do