Change to HttpUtility.ParseQueryString

We experienced an issue with the "fix" that had been applied by @tpluscode in this commit: https://github.com/NancyFx/Nancy/commit/505aa2dda3d4cffe0688cac8aa18fd8c086ce226 which was then released in Nancy v0.23.2.

We use YUI, and its "combo" feature makes requests in the following format:
> http://example.com/foo?file1.js&file2.js&subdir/file3.js

These requests were causing an ArgumentOutOfRangeException in our combo handler.

I fixed the problem and in turn decided to refactor the method entirely as it was very difficult to follow.