into_inner() for CSV Writer

[joshuataylor]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would love an into_inner() for the CSV writer, this is what I have been using:

    pub fn into_inner(self) -> W {
        self.writer.into_inner().unwrap()
    }

In the CSV Writer, and it has been working well for my project. It exists in the JSON Writer.

Describe the solution you'd like
Add the function to CSV writer.

Describe alternatives you've considered
Not sure how else I would access the writer, it exists in the JSON writer.

[alamb]

This would be a great new issue for someone to add ❤️