CSV doesn't offer a way to separate different sheets from each other and
it doesn't really have a standard format. We separate sheets with a
newline so they can be imported into something like Excel or LibreOffice
more easily.
The number of columns for each sheet is inferred from the first returned
row in each statement since they should all be the same.
The `Content-Type` header is meant to say what the content type of the
request is supposed to be and `Accept` is used to ask for a specific
content type. I messed this up and used `Content-Type` instead of
`Accept`. This works with `GET` requests accidentally, but `POST`
requests stop working.