Fix undefined index header.generate-toc in Differential

Summary:
getMarkupEngineDefaultConfiguration() is not setting
header.generate-toc. This is causing "undefined index" in Differential
page.

Test Plan:
  - Differential reviion page renders correctly
  - Phriction page renders correctly

Reviewers: epriestley, tuomaspelkonen, nh

Reviewed By: nh

CC: aran, nh

Differential Revision: https://secure.phabricator.com/D1342

src/applications/markup/engine/PhabricatorMarkupEngine.php

@@ -78,6 +78,7 @@ class PhabricatorMarkupEngine {
       'custom-inline' => array(),
       'custom-block'  => array(),
       'differential.diff' => null,
+      'header.generate-toc' => false,
       'macros'        => true,
       'uri.allowed-protocols' => PhabricatorEnv::getEnvConfig(
         'uri.allowed-protocols'),