* Migrate tests to xunit

* Update sample project to dotnetcore3.1
* Use Sqlite in sample project to run it everywhere
* Fix: Filter with escaped comma
* Fix: Filter "null" does not work with Contains or StartsWith
* Code cleanup: Adjust namespaces, adjust usings

Sieve.Sample/appsettings.Development.json

@@ -0,0 +1,10 @@
+{
+  "Logging": {
+    "IncludeScopes": false,
+    "LogLevel": {
+      "Default": "Debug",
+      "System": "Information",
+      "Microsoft": "Information"
+    }
+  }
+}