Use getPhysicalPath instead of absolute_url_path in api.content.get()

This should fix #152. I haven't added any additional tests because this would be somewhat tricky to set up.. I have tested it manually with nginx/virtual host configuration as per [1].

[1] http://developer.plone.org/hosting/nginx.html#minimal-nginx-front-end-configuration-for-plone-on-ubuntu-debian-linux