Decode SAMLResponse and SAMLRequest as UTF-8 strings

I ran into encoding problems when decoding SAML messages from FEIDE. It turned out that UTF-8 characters were converted into ASCII such that e.g. "ø" became "C8". This patch solves the problem.

The SAML standard states: "The character set of the XML document resulting from decoding the data is determined in the normal fashion, and defaults to UTF-8 if no character set is indicated."

To support encodings other than UTF-8 more work is needed, but I'd suggest using UTF-8 instead of ASCII.