JSON support

The default serializer used to serialize to and from JSON is System.Text.Json.

The serializer is used with the .JsonContent(..) match extension as well as the .RespondJson(..) response extension.

To change the serializer to Newtonsoft for all expectations and verifications on the mock:

var mockHttp = new MockHttpHandler();
mockHttp.UseNewtonsoftJson();

To change the serializer to Newtonsoft for a specific request or response only, pass in the JsonSerializerSettings when setting up the request or response.

The default serializer has changed since v3.x from Newtonsoft to System.Text.Json.