WP_PLUGIN_URL doesn't take SSL into account
see for correct detemition of the plugin url: http://codex.wordpress.org/Determining_Plugin_and_Content_Directories
via: http://wordpress.org/support/topic/plugin-qr-code-tag-not-compatible-with-ssl-https and http://wordpress.org/support/topic/plugin-qr-code-tag-use-protocol-sensitive-urls-for-assets