Adds 'edd_related_downloads_image_size' filter and checks image URL scheme in SSL sites

Adds `edd_related_downloads_image_size` filter to modify the image size retrieved for featured image and checks if the image URL has the correct scheme in SSL environments. The latter is due to this Trac ticket
https://core.trac.wordpress.org/ticket/15928
and since wp_get_attachment_image_src uses image_downsize which uses wp_get_attachment_url.