Fix in Auto-Mapping methods for protected/private properties

For a problem description see: https://groups.google.com/d/msg/servicestack/DYI1LlBG4zo/IB-njmxuFAAJ

As Demis suggested in the answer to my post to the list, I ensured that the Set-Method of the property is accessible. This prevents an ArgumentNullException in PropertyInvoker.

I also included a test case ReflectionExtensionsTests::Can_translate_generic_list_does_ignore_protected_setters that failed without the change. The other existing tests still pass.