Revert "Allow an attribute to be set to no value (None, [], ..) before being assigning a value."

This reverts commit c9f0955.

Author: rohe

src/oidcmsg/configure.py

@@ -105,7 +105,7 @@ def __getattr__(self, item, default=None):
             return default
 
     def __setattr__(self, key, value):
-        if key in self and self.key:
+        if key in self:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
 

src/oidcmsg/context.py

@@ -5,6 +5,7 @@
 from cryptojwt.key_jar import init_key_jar
 
 from oidcmsg.impexp import ImpExp
+from oidcmsg.message import Message
 
 
 def add_issuer(conf, issuer):