I'm certainly glad you found out about that detail. There was a change to the standard made some time ago to clarify the behavior when the TransferProtocol is not specified in the request:
> If this parameter is not provided (or supported), and a transfer protocol is not specified by a scheme contained within this URI, the service shall use HTTP to retrieve the image.
So, newer versions of iDRAC likely have that fallback support to use HTTP when "http://" is being removed from the URI supplied. Still odd that it was working for me with the same iDRAC version. It also would've been much clearer if iDRAC returned a different error message stating something like "ImageURI invalid; scheme needs to be specified".