add Content-Length headers to send_file and send_data for rails 3

Change-Id: I2ace18b38a7ff697312039d1e4eb4b4a5c1b4217
Reviewed-on: https://gerrit.instructure.com/30923
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>