Clean up unit tests so they can be used for the Oracle provider and one ...

...bug fix.

The tests can now be soft linked to the ServiceStack.OrmLite.Oracle.Tests
project.

Fix the test for null updates in OrmLiteWriteExtensions to allow empty strings
as well as null.

Here is the first pull request to clean up the tests so that I can use them for testing the Oracle provider without having to duplicate them. I don't think I broke anything, but I can't test SQL Server from home and I've been at home all week. Some tests don't work, but they didn't work for me before.