test: Add Test Case for Allow Header

The Allow header must be sent if the server responds with 405 to
indicate which request methods can be used.

See cdimascio#467.

test/allow.header.spec.ts

@@ -0,0 +1,58 @@
+import { expect } from 'chai';
+import * as express from 'express';
+import * as request from 'supertest';
+import * as packageJson from '../package.json';
+import { OpenAPIV3 } from '../src/framework/types';
+import { createApp } from './common/app';
+
+describe(packageJson.name, () => {
+  let app = null;
+
+  before(async () => {
+    app = await createApp({ apiSpec: createApiSpec() }, 3001, (app) =>
+      app.use(
+        app.basePath,
+        express
+          .Router()
+          .get(`/pets`, () => ['cat', 'dog'])
+          .post(`/pets`, (req, res) => res.json(req.body)),
+      ),
+    );
+  });
+
+  after(() => {
+    app.server.close();
+  });
+
+  it('adds allow header to 405 - Method Not Allowed', async () =>
+    request(app)
+      .put('/pets')
+      .expect(405)
+      .then((response) => {
+        expect(response.header).to.include({ allow: 'GET, POST' });
+      }));
+});
+
+function createApiSpec(): OpenAPIV3.Document {
+  return {
+    openapi: '3.0.3',
+    info: {
+      title: 'Petstore API',
+      version: '1.0.0',
+    },
+    paths: {
+      '/pets': {
+        get: {
+          responses: {
+            '200': { description: 'GET Pet' },
+          },
+        },
+        post: {
+          responses: {
+            '200': { description: 'POST Pet' },
+          },
+        },
+      },
+    },
+  };
+}