SAML Metadata for serviceproviders should also provide the necessary information

[stebenz]

Is your feature request related to a problem? Please describe.
There is a function to unmarshal the metadata to a serviceprovider but it is not checked with the SPSSODescriptor that the metadata is also part of a serviceprovider.

Describe the solution you'd like
Check if the SPSSODescriptor is not empty with the necessary attributes.

Describe alternatives you've considered
Leave it as is, which could result in errors which are not really descriptive.