Fix validity check of the SAML protocol message receiver endpoint

The base SAML2 message decoder triggers a check to determine if the SAML message's intended destination endpoint matches the actual recipient's endpoint.

The method BaseSAMLMessageDecoder.getActualReceiverEndpointURI relies on HttpServletRequest.getRequestURL to determine the actual recipient URL but this is invalid behind a reverse proxy.

This is a know issue for OpenSAML:

https://issues.shibboleth.net/jira/browse/JOST-65