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

* MockFile.Exists(null) now returns false instead of throwing.
* MockFile.Create(existingFile) now overwrites existing file instead of throwing. 
 * Fixed in MockFileSystem.AddFile()
* MockFile.Create(readOnlyFile) now throws UnauthorizedAccessException. 
 * Implemented MockFile.SetAttributes().