Reuse a unique fake StringIO file for CSV generation

[davidbgk]

No description provided.

[noirbizarre]

r+ if you're sure that seek(0) + truncate(0) is lighter/faster than a new StringIO instance.

[davidbgk]

Actually, it's not faster, I'll update with an explicit del instead ;)

[davidbgk]

I might be onto something, with croquemort it's definitely better to use the truncate/seek pattern otherwise the generator is hanging prior to streaming. Checking.

[davidbgk]

The results are almost the same so I let the explicit StringIO creation but I added the del because I can't test that for sure.