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.
Yes, put it on IProperty. It should be a read-only property, not a method. UnderlyingType is a reasonable name, but I'm open to other suggestions.