Implemented ShouldRenderFile

Please review.

If you are happy with these changes then I am and you can merge. 
As I write this comment I realize that the *code* needs a comment too (and a unit test..)

As you can see this code resets the stream position. 

In reality I did this in response to test pollution (see my other comments) but in practice I also think it makes sense for the test methods not to have any externally visible side effects.