Refs #5138 -- Refactored implementation of __contains__ in HttpRequest introduced in [6097] after a suggestion from Malcolm. Applied a similar refactor for MergeDict and Context which had comparable behavior.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6098 bcc190cf-cafb-0310-a4f2-bffc1f526a37