Add the LocaleSelectorMiddleware.

This ports the behavior of the LocaleSelectorFilter to a PS7 middleware.
I've replaced the implict 'accept all' with the an explicit '*'.
Explicit is generally better than implicit in, and makes it easier to
understand what is going on when reading the code later.

src/I18n/Middleware/LocaleSelectorMiddleware.php

@@ -0,0 +1,61 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.3.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\I18n\Middleware;
+
+use Cake\I18n\I18n;
+use Locale;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+
+/**
+ * Sets the runtime default locale for the request based on the
+ * Accept-Language header. The default will only be set if it
+ * matches the list of passed valid locales.
+ */
+class LocaleSelectorMiddleware
+{
+    /**
+     * List of valid locales for the request
+     *
+     * @var array
+     */
+    protected $locales = [];
+
+    /**
+     * Constructor.
+     *
+     * @param array $locales A list of accepted locales, or ['*'] to accept any
+     *   locale header value.
+     */
+    public function __construct(array $locales = [])
+    {
+        $this->locales = $locales;
+    }
+
+    /**
+     * @param ServerRequestInterface $request  The request.
+     * @param ResponseInterface $response The response.
+     * @param callable $next The next middleware to call
+     * @return \Psr\Http\Message\ResponseInterface A response
+     */
+    public function __invoke(ServerRequestInterface $request, ResponseInterface $response, $next)
+    {
+        $locale = Locale::acceptFromHttp($request->getHeaderLine('Accept-Language'));
+
+        if ($locale && (in_array($locale, $this->locales) || $this->locales === ['*'])) {
+            I18n::locale($locale);
+        }
+        return $next($request, $response);
+    }
+}

tests/TestCase/I18n/Middleware/LocaleSelectorMiddlewareTest.php

@@ -0,0 +1,116 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.3.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Test\TestCase\I18n\Middleware;
+
+use Cake\I18n\I18n;
+use Cake\I18n\Middleware\LocaleSelectorMiddleware;
+use Cake\TestSuite\TestCase;
+use Locale;
+use Zend\Diactoros\Response;
+use Zend\Diactoros\ServerRequestFactory;
+
+/**
+ * Test for LocaleSelectorMiddleware
+ */
+class LocaleSelectorMiddlewareTest extends TestCase
+{
+
+    /**
+     * setup
+     *
+     * @return void
+     */
+    public function setUp()
+    {
+        parent::setUp();
+        $this->locale = Locale::getDefault();
+        $this->next = function ($req, $res) {
+            return $res;
+        };
+    }
+
+    /**
+     * Resets the default locale
+     *
+     * @return void
+     */
+    public function tearDown()
+    {
+        parent::tearDown();
+        Locale::setDefault($this->locale);
+    }
+
+    /**
+     * The default locale should not change when there are no accepted
+     * locales.
+     *
+     * @return void
+     */
+    public function testInvokeNoAcceptedLocales()
+    {
+        $request = ServerRequestFactory::fromGlobals();
+        $response = new Response();
+        $middleware = new LocaleSelectorMiddleware([]);
+        $middleware($request, $response, $this->next);
+        $this->assertSame($this->locale, I18n::locale());
+    }
+
+    /**
+     * The default locale should not change when the request locale is not accepted
+     *
+     * @return void
+     */
+    public function testInvokeLocaleNotAccepted()
+    {
+        $request = ServerRequestFactory::fromGlobals(['HTTP_ACCEPT_LANGUAGE' => 'en-GB,en;q=0.8,es;q=0.6,da;q=0.4']);
+        $response = new Response();
+        $middleware = new LocaleSelectorMiddleware(['en_CA', 'en_US', 'es']);
+        $middleware($request, $response, $this->next);
+        $this->assertSame($this->locale, I18n::locale(), 'en-GB is not accepted');
+    }
+
+    /**
+     * The default locale should change when the request locale is accepted
+     *
+     * @return void
+     */
+    public function testInvokeLocaleAccepted()
+    {
+        $request = ServerRequestFactory::fromGlobals(['HTTP_ACCEPT_LANGUAGE' => 'es,es-ES;q=0.8,da;q=0.4']);
+        $response = new Response();
+        $middleware = new LocaleSelectorMiddleware(['en_CA', 'es']);
+        $middleware($request, $response, $this->next);
+        $this->assertSame('es', I18n::locale(), 'es is accepted');
+    }
+
+    /**
+     * The default locale should change when the '*' is accepted
+     *
+     * @return void
+     */
+    public function testInvokeLocaleAcceptAll()
+    {
+        $response = new Response();
+        $middleware = new LocaleSelectorMiddleware(['*']);
+
+        $request = ServerRequestFactory::fromGlobals(['HTTP_ACCEPT_LANGUAGE' => 'es,es-ES;q=0.8,da;q=0.4']);
+        $middleware($request, $response, $this->next);
+        $this->assertSame('es', I18n::locale(), 'es is accepted');
+
+        $request = ServerRequestFactory::fromGlobals(['HTTP_ACCEPT_LANGUAGE' => 'en;q=0.4,es;q=0.6,da;q=0.8']);
+        $middleware($request, $response, $this->next);
+        $this->assertSame('da', I18n::locale(), 'da is accepted');
+    }
+}