Always return HttpQSCollection (fixes #1132).

Sometimes `HttpUtility.ParseQueryString` is [used as a factory](http://stackoverflow.com/a/1877016/121146) for creating `NameValueCollection` instances that can be serialized back into a query string. Therefore it is valuable to get back a `HttpQSCollection` (which has the useful `ToString` overload) even when the `query` parameter is an empty string.