[Strengthen Encryption] PySAML2 Encrypted Assertions work with Shibboleth SP 3

[peppelinux]

This PR achieved #745

I have been forced for some time to disable assertion encryption due to an incompatibility issue between my pysaml2 based IdP and a Shibboleth SP, with this patch I got it to work.

It solved two weakness in pysaml2 encryption method, that ShibSP highlighted as follows

Fixed: "ERROR Shibboleth.SSO.SAML2 [6] [default]: failed to decrypt assertion: 
Unable to resolve any key decryption keys."

and also

WARN XMLTooling.Decrypter [7] [default]: XMLSecurity exception while decrypting key: 
XSECAlgorithmMapper::mapURIToHandler - URI http://www.w3.org/2001/04/xmlenc#rsa-1_5 
disallowed by whitelist/blacklist policy

Another warning about pysaml2 encryption with Shib SP is the follow:

• saml2.server.create_authn_response have to disable pefim.

encrypt_assertion, encrypt_advice_attributes, encrypt_assertion_self_contained works great instead.

Using PEFIM we'll have, shib SP side, this message

WARN Shibboleth.AttributeResolver.Query [4] [default]: no SAML 2 AttributeAuthority role found in metadata

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