Fixed NullReferenceException behavior in MockDirectory.GetFiles

GetFiles was erroniously returning a NullReferenceException when passing
in a null path. Changed to match Directory.GetFiles behavior which is to
throw an ArgumentNullException.
Merge pull request #85 from bacathey/Fix_Directory_GetFiles_NullReferenceException_behavior

Fixed NullReferenceException behavior in MockDirectory.GetFiles