removing uses of assert() with string value

lib/SimpleSAML/Auth/Default.php

lib/SimpleSAML/Auth/ProcessingChain.php

lib/SimpleSAML/Auth/ProcessingFilter.php

lib/SimpleSAML/Auth/Simple.php

lib/SimpleSAML/Auth/Source.php

lib/SimpleSAML/Auth/State.php

lib/SimpleSAML/Bindings/Shib13/Artifact.php

lib/SimpleSAML/Bindings/Shib13/HTTPPost.php

lib/SimpleSAML/Configuration.php

lib/SimpleSAML/Database.php

lib/SimpleSAML/Error/Assertion.php

lib/SimpleSAML/Error/AuthSource.php

lib/SimpleSAML/Error/BadRequest.php

lib/SimpleSAML/Error/Error.php

lib/SimpleSAML/Error/Exception.php

lib/SimpleSAML/Error/MetadataNotFound.php

lib/SimpleSAML/Error/NotFound.php

lib/SimpleSAML/IdP.php

lib/SimpleSAML/IdP/IFrameLogoutHandler.php

lib/SimpleSAML/IdP/TraditionalLogoutHandler.php