Re-set timeout in ShapedRequest on rate-limit conditions

Summary: Rate-limit conditions didn't set a new timer. It results in stopping of
periodically updating Preview and also in missing last typed characters in
Preview.

Test Plan:
Go to any diff
Type something really fast in Comment
After finishing typing, whole comment should be displayed in Preview
Insert something without keyboard (e.g. paste with mouse)
Preview should be updated

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1288