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