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 +1,85 @@
+import { expect } from 'chai';
+import * as express from 'express';
+import { Server } from 'http';
+import * as request from 'supertest';
+import * as packageJson from '../package.json';
+import * as OpenApiValidator from '../src';
+import { OpenAPIV3 } from '../src/framework/types';
+import { startServer } from './common/app.common';
+
+describe(packageJson.name, () => {
+  let app = null;
+
+  before(async () => {
+    app = await createApp();
+  });
+
+  after(() => {
+    app.server.close();
+  });
+
+  it('adds allow header to 405 - Method Not Allowed', async () =>
+    request(app)
+      .put('/v1/pets/greebo')
+      .expect(405)
+      .then((response) => {
+        expect(response.header).to.include({ allow: 'POST, GET' });
+      }));
+});
+
+async function createApp(): Promise<express.Express & { server?: Server }> {
+  const app = express();
+
+  app.use(
+    OpenApiValidator.middleware({
+      apiSpec: createApiSpec(),
+      validateRequests: true,
+    }),
+  );
+  app.use(
+    express
+      .Router()
+      .get('/v1/pets/:petId', () => ['cat', 'dog'])
+      .post('/v1/pets/:petId', (req, res) => res.json(req.body)),
+  );
+
+  await startServer(app, 3001);
+  return app;
+}
+
+function createApiSpec(): OpenAPIV3.Document {
+  return {
+    openapi: '3.0.3',
+    info: {
+      title: 'Petstore API',
+      version: '1.0.0',
+    },
+    servers: [
+      {
+        url: '/v1/',
+      },
+    ],
+    paths: {
+      '/pets/{petId}': {
+        parameters: [
+          {
+            in: 'path',
+            name: 'petId',
+            required: true,
+            schema: { type: 'string' },
+          },
+        ],
+        get: {
+          responses: {
+            '200': { description: 'GET Pet' },
+          },
+        },
+        post: {
+          responses: {
+            '200': { description: 'POST Pet' },
+          },
+        },
+      },
+    },
+  };
+}