Ignore data-src when src is real image.

[mgdodge]

If both data-src and src are present, and src is set to something other
than holder.js (or other configured domain), leave the image untouched.

Someone using holder.js in a CMS template can then easily set just the src of the image instead of rewriting the entire image tag to override the holder.js fallback. Example:

<?php
// Set image source if available
$src = isset($realimg) ? " src='$realimg'" : ''; ?>
<img data-src="holder.js/500x300"<?php echo $src; ?> />

[mgdodge]

Much more concise. Can't believe I missed that, actually.