[Rails5] Fix sessions_controller_spec

Refactor sessions_controller_spec to work both in rails4 and rails5.
Remove the stubbed `request.referer` method, set real header instead.

changelogs/unreleased/blackst0ne-rails5-expected-search-search-seed_project-got-nil.yml

+---
+title: "[Rails5] Fix sessions_controller_spec"
+merge_request: 19936
+author: "@blackst0ne"
+type: fixed

spec/controllers/sessions_controller_spec.rb

@@ -257,15 +257,15 @@ describe SessionsController do
     end
   end
 
-  describe '#new' do
+  describe "#new" do
     before do
       set_devise_mapping(context: @request)
     end
 
-    it 'redirects correctly for referer on same host with params' do
-      search_path = '/search?search=seed_project'
-      allow(controller.request).to receive(:referer)
-        .and_return('http://%{host}%{path}' % { host: 'test.host', path: search_path })
+    it "redirects correctly for referer on same host with params" do
+      host = "test.host"
+      search_path = "/search?search=seed_project"
+      request.headers[:HTTP_REFERER] = "http://#{host}#{search_path}"
 
       get(:new, redirect_to_referer: :yes)