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.
@ajcvickers Would adding a getter to IProperty, e.g. GetUnderlyingType or GetUnwrappedType, work?