fix zero width for cached images in firefox
fix zero width for cached images in firefox

refs CNVS-47040

test plan:
- create a wiki page with an image
- save it
- load the page with cache disabled
- load the page with cache enabled
- both should show the image

Change-Id: I20725690030942b3225b227c097465e61b098789
Reviewed-on: https://gerrit.instructure.com/210893
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Brent Burgoyne <bccf16d7abdd2611580b1cd7d7a6fcc150d619da@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms