Patch definition of WP_CONTENT_URL to include SSL when it might not

This is almost useless. WP_CONTENT_URL is a deprecated constant only used in one theme, elegant-grunge. The definition is based on the output get_option('siteurl'), which I think should also be set to the https address. Nonetheles, just to be safe, let's keep this patch for the moment, but we should check if we can drop it.