Save as menu may cause removing a file

[Danil Sokolov]

To reproduce:
 - Create a model and save it as a.work.
 - Chose File->Save as... and try to overwrite the same file a.work.
 - Close Workcraft, do not save the work when asked.

Observed behaviour:
 - Exception is thrown and file is not saved.
 - After Workcraft is closed without saving the work, the file a.work disappears.

Expected behaviour:
 - The file should be saves the same way as if File->Save was chosen.