Added test project and fixed bug when config values are missing.

This is to fix the bug whereby the library raises a NullReferenceException when a Load is performed and there are values missing from the AppSettings source.

Now, when config values are missing, the library will ignores those properties of the target Settings object and it will retain any previous values it had (the default values for that class if it is newly instantiated).

Also added a test project to illustrate this scenario and prove the fix works.