Allow an attribute to be set to None before being assigning a value.

Author: rohe

src/oidcmsg/configure.py

@@ -105,12 +105,12 @@ def __getattr__(self, item, default=None):
             return default
 
     def __setattr__(self, key, value):
-        if key in self and self.key:
+        if key in self and self.key is not None:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
 
     def __setitem__(self, key, value):
-        if key in self:
+        if key in self and self.key is not None:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)