[patch] Context should have the __contains__ method

[Antti Kaihola]

The Context class in django.template.context should have a __contains__ method defined, so that one can do:

if 'missiles' not in context:
    raise ImproperlyConfigured, \
        'Please include missiles in the context so we can destroy the world.'
    destroy_world_with(context['missiles'])

Although, the destroy_world_with function is left as an exercise for the reader.

A simple patch which concerns context.py is included.

[Antti Kaihola]

Adds the contains method for Context

[Malcolm Tredinnick]

(In [4480]) Fixed #3071 -- added a __contains__ method to the Context class. Thanks
akaihola.