So the answer from the Discourse developers is to "submit a pull request to change the behavior if you believe it can be improved"... which is fair enough really, however, having never written a line of Ruby in my life I'm not best positioned to do this...
Anyone else fancy working out how to prevent Discourse generating HTML that causes mixed content issues? I think it was this pull request which added the feature and it would need a check adding to ensure that a
og:image URL in a remote web page, for example:
<meta property="og:image" content="http://www.progcode.co/img/social-media-1.png" />
Returned an image, rather than a HTML page (as this dose after a redirect), more detail on the thread at meta.discourse.org.