Fix a bug in the imageproxy controller

Summary: Somehow this got through last week :( It's a bug that causes the controller to... *ahem*... just not work. Luckily nothing uses this yet so nothing was really affected.

Test Plan: Hit /file/imageproxy/?uri=http://i.imgur.com/nTvVrYN.jpg and are served a nice picture of a bird

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, yelirekim

Differential Revision: https://secure.phabricator.com/D16598