DataContractJsonSerializer uses default schema when serializing OutboxRecord and used suffix `k__BackingField`

[ramonsmits]

DataContractJsonSerializer uses default schema when serializing OutboxRecord

This results in weird field names that are similar to <Headers>k__BackingField instead of a good schema name that would use the name Headers

[
    {
        "<Headers>k__BackingField": {
            "NServiceBus.MessageId": "860c26ec-d400-49ec-a73c-ad9800cf8d1c",
            "NServiceBus.MessageIntent": "Send",
            "NServiceBus.RelatedTo": "50f4ed5f-67dc-4655-a7ad-ad9800cce246",
            "NServiceBus.ConversationId": "72647d2c-56a0-44a3-8978-ad9800cce248",
            "NServiceBus.CorrelationId": "50f4ed5f-67dc-4655-a7ad-ad9800cce246",
            "NServiceBus.ReplyToAddress": "HeadersContainSpecialCharacters.OutboxEndpoint",
            "NServiceBus.OriginatingMachine": "ZOLDER",
            "NServiceBus.OriginatingEndpoint": "HeadersContainSpecialCharacters.OutboxEndpoint",
            "$.diagnostics.originating.hostid": "ef7f782c7b29ad3e524a28b0fcf0340b",
            "NServiceBus.ContentType": "text\/xml",
            "NServiceBus.EnclosedMessageTypes": "NServiceBus.AcceptanceTests.Outbox.When_headers_contain_special_characters+SendOrderAcknowledgement, NServiceBus.NHibernate.AcceptanceTests, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null",
            "NServiceBus.Version": "8.0.0",
            "NServiceBus.TimeSent": "2021-09-03 12:35:40:257771 Z"
        },
        "<MessageId>k__BackingField": "860c26ec-d400-49ec-a73c-ad9800cf8d1c",
        "<Message>k__BackingField": [
            60,
            63,
            120,
            109,
            108,
            //---
       ],
        "<Options>k__BackingField": {
            "Destination": "HeadersContainSpecialCharacters.OutboxEndpoint"
        }
    }
]