Detect domains correctly in Director sub-calls

Previously it relied on the PHP-level $_SERVER variable; now it will use
the HTTPRequest so it works correctly in more situations.

src/Middleware/InitStateMiddleware.php

@@ -94,7 +94,7 @@ class InitStateMiddleware implements HTTPMiddleware
             return (int) $request->getSession()->get('SubsiteID');
         }
 
-        $subsiteIdFromDomain = Subsite::getSubsiteIDForDomain();
+        $subsiteIdFromDomain = Subsite::getSubsiteIDForDomain($request->getHost());
         if ($subsiteIdFromDomain !== null) {
             return (int) $subsiteIdFromDomain;
         }

tests/php/InitStateMiddlewareTest.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace SilverStripe\Subsites\Tests;
+
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Core\Injector\Injector;
+use SilverStripe\Subsites\Middleware\InitStateMiddleware;
+use SilverStripe\Subsites\Model\Subsite;
+use SilverStripe\Subsites\State\SubsiteState;
+
+class InitStateMiddlewareTest extends BaseSubsiteTest
+{
+    protected static $fixture_file = 'SubsiteTest.yml';
+
+    /**
+     * Original value of $_REQUEST
+     *
+     * @var array
+     */
+    protected $origServer = [];
+
+    protected function setUp()
+    {
+        parent::setUp();
+
+        $this->origServer = $_SERVER;
+    }
+
+    protected function tearDown()
+    {
+        $_SERVER = $this->origServer;
+
+        parent::tearDown();
+    }
+
+    public function testDomainDetectionViaServerHeaders()
+    {
+        $_SERVER['HTTP_HOST'] = 'one.example.org';
+
+        $this->getMiddleware()->process($this->getRequest(), $this->getCallback());
+
+        $expectedSubsite = $this->objFromFixture(Subsite::class, 'domaintest1');
+        $this->assertEquals($expectedSubsite->ID, $this->getState()->getSubsiteId());
+    }
+
+    public function testDomainDetectionViaRequestOverridesServerHeaders()
+    {
+        $_SERVER['HTTP_HOST'] = 'one.example.org';
+
+        $this->getMiddleware()->process($this->getRequest('two.mysite.com'), $this->getCallback());
+
+        $expectedSubsite = $this->objFromFixture(Subsite::class, 'domaintest2');
+        $this->assertEquals($expectedSubsite->ID, $this->getState()->getSubsiteId());
+    }
+
+    protected function getMiddleware()
+    {
+        return new InitStateMiddleware();
+    }
+
+    protected function getRequest($domain = null)
+    {
+        $request = new HTTPRequest('GET', '/test/url');
+        if ($domain) {
+            $request->addHeader('host', $domain);
+        }
+        return $request;
+    }
+
+    protected function getCallback()
+    {
+        return function () {
+        };
+    }
+
+    protected function getState()
+    {
+        return Injector::inst()->get(SubsiteState::class);
+    }
+}