Phriction - fix edits with subscribers

Summary: pre-patch, these fatal, since we overwrite $content to be just a string so methods fail later in the code. Instead, write a $content_str to keep $content as the proper data.

Test Plan: editing a document and on save it showed me the view page! (as opposed to fataling and staying on the eidt page)

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10426
Merge branch 'master' of github.com:facebook/phabricator