New collection type for Url.QueryParams

[tmenier]

Related to #541

Url.QueryParams has always been backed by an IDictionary<string, object>. While very handy in most cases, inevitably the problem came up of how to deal with this perfectly legitimate scenario:

x=1&x=2&x=3

In order to not introduce a breaking change at the time, I took advantage of the fact that the values are objects and simply returned an array for url.QueryParams["x"]. Checking if it's an array is not only clunky, it's also easy to forget, which could lead to bugs. The new collection types introduced in #541 are better suited for this.

What's breaking

Instead of this:

var x = url.QueryParams["x"]; // is it an array? better check!

You will now do this:

var x = url.QueryParams.FirstOrDefault("x"); // always returns a single value
var xs = url.QueryParams.GetAll("x"); // always returns a collection

With QueryParams["x"] gone it's obviously not writeable anymore either. Use QueryParams.AddOrReplace instead.

What's not breaking

The most typical action involving query params is writing them using fluent methods directly on Url:

url.SetQueryParam
url.SetQueryParams
url.RemoveQueryParam
url.RemoveQueryParams

None of these are changing in semantics or behavior.