System.Text.Json support via new ReturnsJsonResponse methods

[maxkagamine]

Long-overdue System.Text.Json extensions!

The new ReturnsJsonResponse methods (named so due to being otherwise ambiguous) are functionally similar to the System.Net.Http.Json extensions, PostAsJsonAsync() etc., and send a JsonContent instead of StringContent.

Example usage (see ResponseExtensionsTests):

List<Song> expected = /* ... */;

handler.SetupRequest(HttpMethod.Get, "https://example.com/api/songs")
    .ReturnsJsonResponse(expected);

var actual = await client.GetFromJsonAsync<List<Song>>("api/songs");

actual.Should().BeEquivalentTo(expected);

RequestExtensionsTests has also been updated with examples of using System.Text.Json to match requests:

handler
    .SetupRequest(HttpMethod.Post, url, async request =>
    {
        var json = await request.Content.ReadFromJsonAsync<Model>();
        return json.Id == expected.Id;
    })
    .ReturnsResponse(HttpStatusCode.Created);

Or if you know the code under test is using the System.Text.Json extensions:

handler
    .SetupRequest(HttpMethod.Post, url, r => ((JsonContent)r.Content).Value == model)
    .ReturnsResponse(HttpStatusCode.Created);

This pairs especially well with record types.