Allow JSON deserialization of HttpTrace

[ferstl]

We need to use a custom HttpTraceRepository to persist all HTTP requests and responses for auditing reasons.

Persisting HttpTrace objects is not a problem. However, recreating them (HttpTraceRepository.findAll()) is not possible in custom HttpTraceRepository implementations due to the package-private constructors in HttpTrace and its inner classes.

This PR does:

• add a public no-arg constructor to HttpTrace
• add public setters for timestamp, request and principal
• add public constructors for the inner classes of HttpTrace
• add @JsonCreator and @JsonProperty annotations for proper deserialization
• add a test for JSON serialization and deserialization

[wilkinsona]

Thanks for the proposal. I’m not keen on opening up the HttpTrace API like this. We don’t intend HttpTrace instances to be created from JSON only serialised to JSON. Making things like the timestamp mutable is also problematic.

Can we please take a step back? Can you open an issue that describes what you’d like to do? I think you want to be able to persist and recreate HttpTrace instances on the server-side, although you haven’t said that in so many words. Once we fully understand the problem we can hopefully agree on some changes that solve that problem.

[ferstl]

Ok, I'll do that...