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