Turn unit tests back on

A fake `BindingContext` was added like the tests in [JsonBodyDeserializerFixture](https://github.com/NancyFx/Nancy/blob/34d88eb387d152e93b37600fe937900a278c88bd/src/Nancy.Tests/Unit/ModelBinding/DefaultBodyDeserializers/JsonBodyDeserializerFixture.cs)

The file name doesn't match the class name (casing difference) but I left that alone for now.