TST add test for Memory.clear when cachedir is None
TST add test for Memory.clear when cachedir is None

See #200.

joblib/joblib,karandesai-96/joblib,aabadie/joblib,lesteve/joblib,karandesai-96/joblib,tomMoral/joblib,aabadie/joblib,joblib/joblib,tomMoral/joblib,lesteve/joblib