Northwind Tests for Redis

Implemented the Northwind tests for Redis plus some tidying up (e.g. removed BaseClassFixture since not really using it).

Implementing these tests meant implementing RedisQueryModelVisitor.BindMethodCallToValueReader() plus updating some null handling in RedisDatabase.
Seems like this would be useful in metadata, @ajcvickers? 

Either way, you should use SharedTypeExtensions.UnwrapNullableType.