RFile; added property .closed to mirror standard `file` behaviour.

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-649 into branch/default

Created originally on Bitbucket by Bystroushaak (Lishaak Bystroushaak)

Was already merged in Bitbucket before import, marked as merged by the import user

I've added .closed property which allows you to check whether RFile instance is closed without the need to use _check_closed() which raises ValueError if the file is closed.

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