add test to known if has a value + some corrections

tiramisu/value.py

@@ -92,10 +92,10 @@ class Values(object):
         :param opt: the `option.Option()` object
         """
         path = self._get_opt_path(opt)
-        self._contains(path)
+        return self._contains(path)
 
     def _contains(self, path):
-        return self._p_.hasvalue('value', path)
+        return self._p_.hasvalue(path)
 
     def __delitem__(self, opt):
         """overrides the builtins `del()` instructions"""