Add spec for security issue

We want to make sure that response is consistent so that
we prevent brute force attacks that could expose project
existence

spec/requests/projects/blob_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::BlobController do
+  let(:project) { create(:project, :private, :repository) }
+  let(:namespace) { project.namespace }
+
+  context 'anonymous user views blob in inaccessible project' do
+    context 'with default HTML format' do
+      before do
+        get namespace_project_blob_path(namespace_id: namespace, project_id: project, id: 'master/README.md')
+      end
+
+      context 'when project is private' do
+        it { expect(response).to have_gitlab_http_status(:redirect) }
+      end
+
+      context 'when project does not exist' do
+        let(:namespace) { 'non_existent_namespace' }
+        let(:project)   { 'non_existent_project' }
+
+        it { expect(response).to have_gitlab_http_status(:redirect) }
+      end
+    end
+
+    context 'with JSON format' do
+      before do
+        get namespace_project_blob_path(namespace_id: namespace, project_id: project, id: 'master/README.md', format: :json)
+      end
+
+      context 'when project is private' do
+        it { expect(response).to have_gitlab_http_status(:unauthorized) }
+      end
+
+      context 'when project does not exist' do
+        let(:namespace) { 'non_existent_namespace' }
+        let(:project)   { 'non_existent_project' }
+
+        it { expect(response).to have_gitlab_http_status(:unauthorized) }
+      end
+    end
+  end
+end