SSL / HTTPS fix for Simple Portfolio plugin

To make the Simple Portfolio plugin (https://wordpress.org/plugins/simple-portfolio/) working via https (or with a ssl certificate) you must change some files in the source code.

Edit the following files:

• /extends/admin-head.php
• /extends/admin-menu.php
• /extends/init.php
• /extends/save-post.php

and replace

WP_PLUGIN_URL

with

plugins_url()

Discussion here: https://wordpress.org/ideas/topic/wp_plugin_url-doesnt-take-ssl-into-account