Use JsonSettings for date formatting and casing

This fixes https://github.com/NancyFx/Nancy/issues/1607

I wasn't sure how we wanted the settings from  `JsonConvert.DefaultSettings()` and `JsonSettings` to take priority so that might need to be revised a bit.

It'd be helpful if [Json.IsJsonContentType()](https://github.com/NancyFx/Nancy/blob/45c2cc1f07c21081c00e6a40e79c468a3c9dba7c/src/Nancy/Json/Json.cs#L111) could be made public since all of the serializers have their own copy which is out of sync.