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.
OK. UnderlyingType it is. If no-one objects I'll remove the SharedTypeExtension UnwrapNullableType() method at the same time and make everything point to UnderlyingType instead.