local storage thumbnail https

Thumbnails should come over https if the page is being viewed over https.

Change-Id: Id934c3ce3668cc5a72d90ee01e632bfe6dd0515b
Reviewed-on: https://gerrit.instructure.com/17053
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>

app/models/thumbnail.rb

@@ -31,7 +31,7 @@ class Thumbnail < ActiveRecord::Base
                    :keep_profile => true
 
     def authenticated_s3_url(*args)
-      "http://#{HostUrl.context_host(attachment.context)}/images/thumbnails/show/#{id}/#{uuid}"
+      "#{HostUrl.protocol}://#{HostUrl.context_host(attachment.context)}/images/thumbnails/show/#{id}/#{uuid}"
     end
   else
     has_attachment :content_type => :image,