Implemented ShouldRenderFile

Please review.
I suspect this (my code) to be a good idea because we are going to be operating on the *result* of an operation. 

The action method could very well return a stream whose position is zero (and therefore valid at execution time) but as soon as the developer calls into our library, the position of the stream might change and cause subsequent tests to behave badly.

Responding to both this comment and your comment about test pollution: Just comment the relevant line and you will see what happens. I want to know your thoughts anyway.