Added fallback store feature

tests/OfflineTests/ObjectRetrieverTest.php

@@ -0,0 +1,29 @@
+<?php
+
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+namespace CloudObjects\SDK;
+
+use CloudObjects\SDK\Exceptions\InvalidSDKConfigurationException;
+
+class ObjectRetrieverTest extends \PHPUnit\Framework\TestCase {
+
+    public function testFallbackStoreWithExistingDirectory() {
+        $retriever = new ObjectRetriever([
+            'fallback_store_directory' => sys_get_temp_dir(),
+            'fallback_store_scope' => '*',
+        ]);
+        $this->assertInstanceOf(ObjectRetriever::class, $retriever);
+    }
+
+    public function testFallbackStoreWithMissingDirectory() {
+        $this->expectException(InvalidSDKConfigurationException::class);
+        new ObjectRetriever([
+            'fallback_store_directory' => sys_get_temp_dir().'/does-not-exist-'.uniqid(),
+            'fallback_store_scope' => '*',
+        ]);
+    }
+
+}