Bugfix: Respect customizations on JsonSerializerSettings

[bart-degreed]

Bugfix: Respect customizations on JsonSerializerSettings when serializing response.

Fixes #687.

[bart-degreed]

I've come to realize that JsonSerializerSettingsNullValueHandlingScope was a very bad idea: Changing options on the shared instance will likely affect concurrent requests. As described here, locking is not going to help.

The right way to deal with this is to have the settings copied into a new serializer instance. There is no Clone method, but calling JsonSerializer.Create(settings) does perform the copy internally. As an optimization, we may even cache the JsonSerializer instance (it is thread-safe).

[bart-degreed]

Replaced by #720.