Use correct mime type for message content type

[Mike Minutillo]

In the serializer, there is this line which sets the message content type to be "json". This should be a mime type. The correct value is "application/json". The best way to do this is to use ContentTypes.Json which can be found in NServiceBus core

Unfortunately, ServiceControl is unable to parse the content type "json", which prevents showing message bodies in ServiceInsight or ServicePulse.