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.
Add curles to if body's even if they contain only one line :smile: