Probe only checks HTTP response of IdP to determine metadata consumption

[alexstuart]

Works on v3 Shibboleth IdP (HTTP 200 is has metadata, 400/500 if not). Doesn't work on, for example, simpleSAMLphp which sends HTTP 200 & error text in HTML page. Nor on IdPs which give a login box whether or not the ACS URL is not in metadata.