MockFile.Exists(null) now returns false instead of throwing.
MockFile.Create(existingFile) now overwrites existing file instead of throwing. Fixed in MockFileSystem.AddFile(existingFile).
MockFile.Create(readOnlyFile) now throws UnauthorizedAccessException. Implemented MockFile.SetAttributes.
Merge pull request #41 from aklg/master

Fixed MockFile.Exists(null), MockFile.Create(existingFile) and MockFile.Create(readOnlyFile)