Don't check what's returned from passed function

1. By default, CoffeeScript will return the last line in a function. If you had the following:
    ```coffeescript
    .pipe($.rename (path) =>
            path.extname = '.html'
    )
    ```

   Then your gulp task would fail with `TypeError: Arguments to path.join must be strings`, unless you end the function with `return path` / `return` / `return` a falsy value.
   
2. It's kinda ambiguous / confusing. I can't think of a use case where you'd edit the original but return something else (since the original will overridden with what you return anyway).

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I didn't check the tests, I just made this change on github.com. If they fail, I'll come back to them later.