Make user access test pass for the right reason

If we do not set a private token during the test, current_user will be
nil because the user is not found, not due to the access check.

spec/requests/api/api_helpers_spec.rb

@@ -45,6 +45,7 @@ describe API, api: true do
     end
 
     it "should return nil for a user without access" do
+      env[API::APIHelpers::PRIVATE_TOKEN_HEADER] = user.private_token
       Gitlab::UserAccess.stub(allowed?: false)
       current_user.should be_nil
     end