fix wiki_page_importer only_when_blank setting overwriting

also protect the class level variables set by acts_as_url
from being overwritten in the future

test plan:
* running spec/apis/v1/content_exports_api_spec.rb and
 spec/apis/v1/pages_api_spec.rb together in that order
 should not cause spec failures

Change-Id: I7c6cde252b2780365ae05438ef4f56135c6edd2f
Reviewed-on: https://gerrit.instructure.com/40080
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>