as_dbc_string() database's method loses the extended flag about cantools HOT 4 CLOSED

The as_-methods are not the primary feature of this package, and should probably be removed since they are not fully functional. Either that, or I should add a limitation in the README notifying the user about their flaws. That said, feel free to implement the missing feature if you find it useful.

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It seems, that exactly for that issue, it would be easy set a high bit for the extended frame in file cantools/db/formats/dbc.py for each case where is used message.frame_id, something like this

    ...
    msg.append(fmt.format(frame_id=message.frame_id | 1<<31 if message.is_extended_frame else 0,
    ...

Instead of removing the working functionality.

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Feel free to create a pull request with the change and I'll review and later merge it. Make sure to modify/add tests to ensure the issue is not reintroduced in the future.

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Part of release 20.9.0.

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