Removing hard requirement on Apache in webform_dupe_cop.

The webform_dupe_cop_fetch_headers() function was using apache_request_headers() and apache_response_headers(), and this will cause a WSOD in non-Apache environments.
Merge pull request #492 from JacksonRiver/7.x-webform_dupe_cop-nginx

Removing hard requirement on Apache in webform_dupe_cop