Fix PhutilReadableSerializer to quote strings.

Summary: See D9622#inline-37279 for explanation.

Test Plan: Modified and ran unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9632