Fixed undisposed writers in CSV saving

[daniel-chambers]

This fixes a small bug where the CSV Save functions don't dispose the writers they use, and therefore leave stream flushing and potentially file handles open until garbage collection cleans up the writers. I've simply changed the let writers to use writers to ensure they are disposed correctly.

One point that's worth some discussion: for the Save(stream : Stream ... overload, I've made it so that the save does not close the underlying stream. Instead, the writer flushes into it, but leaves it open. I chose to do this because I'm of the opinion that if I give a stream to something, I (the caller) own it and I will close and dispose it when I'm done with it. I don't expect Save to close and dispose it for me.
However this is not the default behaviour when you pass a Stream to a Writer, hence why I'm using a more complicated overload.

[dsyme]

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