RSS block: "Invalid array key 'url'" messages when using an atom feed

A client reported that when using the external feed block to display the Atom feeds of Mahara's own blogs, you'd see this warning message:

[WAR] f9 (blocktype/externalfeed/lib.php:464) Illegal string offset 'url'
Call stack (most recent first):

    log_message("Illegal string offset 'url'", 8, true, true, "/home/aaronw/www/mahara/htdocs/blocktype/externalf...", 464) at /home/aaronw/www/mahara/htdocs/lib/errors.php:439
    error(2, "Illegal string offset 'url'", "/home/aaronw/www/mahara/htdocs/blocktype/externalf...", 464, array(size 2)) at /home/aaronw/www/mahara/htdocs/blocktype/externalfeed/lib.php:464
    PluginBlocktypeExternalfeed::make_feed_image_tag("http://trinity-mahara.testing.elearning.catalyst.n...") at /home/aaronw/www/mahara/htdocs/blocktype/externalfeed/lib.php:98
    PluginBlocktypeExternalfeed::render_instance(object(BlockInstance)) at Unknown:0
    call_user_func_array(array(size 2), array(size 1)) at /home/aaronw/www/mahara/htdocs/lib/mahara.php:1578
    call_static_method("PluginBlocktypeExternalfeed", "render_instance", object(BlockInstance)) at /home/aaronw/www/mahara/htdocs/blocktype/lib.php:782
    BlockInstance->render_viewing() at /home/aaronw/www/mahara/htdocs/lib/view.php:2049
    View->build_column(1, 1, false) at /home/aaronw/www/mahara/htdocs/lib/view.php:2006
    View->build_columns(1, false) at /home/aaronw/www/mahara/htdocs/lib/view.php:1991
    View->build_rows() at /home/aaronw/www/mahara/htdocs/view/view.php:257

On looking at the code, it looks like the problem is that the PluginBlocktypeExternalfeed::make_feed_image_tag() function is in dire need of a cleanup. It is built to handle an array if the feed is an RSS feed, and a URL string if the feed is an Atom feed. But it just checks for array keys directly, rather than checking whether it's an array or if the array key exists, and although that works, it throws a warning message.