Fix exclusion of favicon.ico and robots.txt on Windows.

This fixes a bug where the exclusion of favicon.ico and robots.txt was not working on Windows.  (That it, on Windows, fetching e.g. /robots.txt would retrieve the app html, when it shouldn't).

The use of path.sep is incorrect because req.url is a URL, not a file
path, and so always contains forward slashes.

I verified that the original code does not work on Windows and that
the new code does.

See https://github.com/TomWij/meteor/issues/50 for discussion.