Fix Diigo API Calls

fixes CNVS-2467

test plan
 - verify you can set up the diigo plugin with the information in gollum https://gollum.instructure.com/OtherServiceTestAccounts
 - on the user settings page, verify you can use your diigo username and password to register the service

Change-Id: I8d33a8c1c29a013c681488fcc38bbfe7b83cd398
Reviewed-on: https://gerrit.instructure.com/39942
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>