MAGETWO-36972: Expose CMS api's as web API

Author: AndriyNasinnyk

dev/tests/api-functional/testsuite/Magento/Cms/Api/PageRepositoryTest.php

@@ -123,7 +123,7 @@ public function testCreate()
             ],
         ];
 
-        $requestData = ['page' => [
+        $requestData = ['id', 'page' => [
                 PageInterface::IDENTIFIER => $pageDataObject->getIdentifier(),
                 PageInterface::TITLE      => $pageDataObject->getTitle(),
             ],