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.
@anpete I looked at that. But it's internal. If I try to make it public I get clashes since it's included in both EntityFramework and EntityFramework.InMemory (and similarly for EntityFramework.Redis). We could make it public and move it into EF so that it's only defined once?