Mocking a HttpContext

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MVC4

unit-testing

My application has a BaseController in which the HttpContext.Current.Session[CacheKey] is used.
how do i mock the above session while unit testing.
I have the fake HttpContext Helpers...how do i use them....
also my HttpContext.Current.Session[CacheKey] manages the language of the browser where CacheKey is:

public static readonly string CacheKey = "en-us";