fix files controller for rails 3 specs

 * don't use respond_to if json is the only possible format
 * when filtering scribd_doc, rails 3 serializes as a hash with
   string keys, as opposed to a HashWithIndifferentAccess, so
   delete with string key
 * don't use StringIO as an uploaded file; Rails 3 TestCase will
   call to_param on it
 * don't require the .:format part of the public_url route

Change-Id: I282f529f1ee0941b52a996eefa01647a018f70d3
Reviewed-on: https://gerrit.instructure.com/29930
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>