I recommend forcing it. As far as I'm aware, forcing it will simply just skip the content-type check, but any parse errors will still occur if it can't be parsed.
Of course, the real solution would be for the site to change to an actual valid content-type for RSS, but I doubt that can happen. As far as I can see, there's no point in adding an upstream patch to SP. I'm fairly sure that the RSS specification, somewhere, mentions that a correct MIME type must be used, though I can't find this.