JsonSerializer.writeValue should accept immutable/const values

[japplegame]

No description provided.

[s-ludwig]

Since T is a template type, it should usually be inferred to const/immutable just fine. If you have any failing case, can you add that as a unit test?

[japplegame]

serialization fails if immutable / const class has toJson / fromJson methods.
Caused by this line: https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/data/serialization.d#L308
Perhaps it will be better to change serialization.d instead json.d:
serializer.writeValue!TU(value) -> serializer.writeValue!T(value).

[s-ludwig]

Okay, I've taken another look and I think this is the safest fix. I'll merge and add a unit test.