Fixed the signing procedures that calls xmlseclibs, to correctly set the...

Fixed the signing procedures that calls xmlseclibs, to correctly set the exclusive canonicalization parameter XMLSecurityDSig::EXC_C14N. After tips from the xmlseclib author Rob Richards. Would be great if Hans Z. tested this afterwards as he is using a php version before 5.2, and that was the only place where this problem occured.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@177 44740490-163a-0410-bde0-09ae8108e29a