don't calculate the fingerprint for anything that is not an x509 certificate; this fixes an issue where a key value is included -after- the certificate value in the authnresponse and the fingerprint would be overridden (and set to a null value)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@251 44740490-163a-0410-bde0-09ae8108e29a