Empty structs were being serialized as `null`, when they should have been serialized as `{}`.
This was due to the type inference on the serializer - where no calls to `field()` would result in the default `null` type.
To solve this, the `serialize(const void* object, const std::initializer_list<Field>&)` inline helper has been promoted to a virtual function (and renamed to `fields()`).
The JSON serializer implementation of this now first sets the object type to `object`, even if there are no fields to serialize.
Added test for this.
Fixes: #10
The `body` field of the Response is optional. Do not error if it is missing.
If the typeinfo of the response. This was incorrectly using the Request type.
Authored by kuafuwang, squashed by ben-clayton.
