The query killing functionality depends on the ResponseWriter exposing a
CloseNotify method. Since we wrap the http.ResponseWriter, the new
struct does not have that method and the HTTP handler would skip past
calling that method.
Instead of duplicating `Flush()` and `CloseNotify()` for every response
formatter, we will unify all of that under a single struct and create
formatters instead.
Also, fixes a bug where the header information from a query would not be
returned until some other data was returned with it because of
buffering and another bug in the gzipResponseWriter that wouldn't flush
the actual underlying ResponseWriter.
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.
Jonathan A. Sternberg
Ben Johnson