The logic to update the request's RequestUri is somewhat old:
518ac1bf19
but it makes sense to do so.
Some testing with a console app, also revealed that a plain .NET does this in
case of a redirect:
```cs
async static Task Main ()
{
var client = new HttpClient () { BaseAddress = new Uri("https://httpbin.org") };
var request = new HttpRequestMessage (HttpMethod.Get,
"/redirect-to?url=https%3A%2F%2Fmicrosoft.com&status_code=302&queries[]={}"
);
Console.WriteLine ($"Request uri 1: {request.RequestUri}");
var response = await client.SendAsync(request);
Console.WriteLine ($"Request uri 2: {request.RequestUri}");
var content = await response.Content.ReadAsStringAsync();
Console.WriteLine ((int) response.StatusCode);
Console.WriteLine(content.Length);
}
```
prints:
```
Request uri 1: /redirect-to?url=https%3A%2F%2Fmicrosoft.com&status_code=302&queries[]={}
Request uri 2: https://www.microsoft.com/es-es/
200
201252
```
Further looking into .NET's code, `SocketsHttpHandler` updates RequestUri after
following a redirect:
f5eb26e4da/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RedirectHandler.cs (L65-L66)
So it seems the expected behavior is to update RequestUri only in case of a
redirect.
Now the question becomes: how to detect whether we're a redirect or not?
Contrary to `SocketsHttpHandler`, we don't do the actual redirect, it's
handled transparently by `NSUrlSession`.
Fortunately `NSUrlSessionTask` has two properties which help:
[`OriginalRequest`][1] and [`CurrentRequest`][2]. According to Apple's
documentation, these are the same, "except when the server has responded to
the initial request with a redirect to a different URL."
This sounds perfect for us, so use these two properties to determine whether a redirect occurred.
Note that we can't use object identity, because these are 'copy' properties,
which means they'll never be the same instances, only at most a copy of
eachother. So instead compare the `AbsoluteString` property to check for
equality. As far as I'm aware, none of the other properties (request body,
headers, cookies, etc.) can be set by the server in a redirect, so there's no
need to compare those.
Fixes https://github.com/xamarin/xamarin-macios/issues/20629.
[1]: https://developer.apple.com/documentation/foundation/nsurlsessiontask/1411572-originalrequest
[2]: https://developer.apple.com/documentation/foundation/nsurlsessiontask/1411649-currentrequest
59e84a5e6c