bugfix: Allow attributes to contain raw XML as their values.

A recent change in simplesamlphp/saml2#60 made the library return a DOMNodeList object when the contents of the AttributeValue element are not text. This lead to a bug, since the returned value is not serializable, and when storing it in the session it will go away as soon as we serialize the session to store it in the backend (whatever that is). This is always, as the SP will always redirect to the URL originating authentication. The result was an empty DOMNodeList object where there should be some value.

This commit makes the SimpleSAML_Session to implement the Serializable interface. When obtaining the attributes during login (doLogin() method), the code will now look for DOMNodeList objects, and dump them as a string with the XML representation of their contents in the 'RawAttributes' array inside $this->authData[$authority]. This allows us to parse the XML back when unserializing, and restore the original DOMNodeList object as the value of the attribute.

The issue was reported originally in the mailing list by Enrico Cavalli, affecting eduPersonTargetedID. This resolves #424.