[#1635] user_dictize() don't crash if no context['user']

Normally 'user' would always be in context (and would be None if no user
was logged in) but if user_dictize() was called via an internal function
calling a logic action function via get_action(), and that internal
function neglected to put 'user': None in context, then user_dictize()
would raise KeyError. (An example of this is the
get_and_send_notifications_for_all_users() that I'm about to commit
after this.)

May as well just behave as if 'user': None was in context instead.

ckan/lib/dictization/model_dictize.py

@@ -399,7 +399,7 @@ def user_dictize(user, context):
     result_dict['number_of_edits'] = user.number_of_edits()
     result_dict['number_administered_packages'] = user.number_administered_packages()
 
-    requester = context['user']
+    requester = context.get('user')
 
     if not (ckan.authz.Authorizer().is_sysadmin(unicode(requester)) or
             requester == user.name or