Comment 1:

You might consider hijacking the connection and writing the HTTP response yourself,
although that would defeat keepalive.
http://golang.org/pkg/net/http/#Hijacker
Would you mind filing a separate issue about the bug?

Comment 2:

Connection: close should only be sent if you are sending a HTTP/1.1 response and have
asked to close the connection after the response has been sent. It should not be sent
for HTTP/1.0 responses, or HTTP/1.1 responses that do not request a close action.

Comment 3:

If I understand correctly, if I hijack the connection, then I need to take care of
composing and emitted well-formed HTTP, no? I can't just use net/http's Request.Write
(because it'll emit these headers), but the implementation of Request.Write relies on a
good deal of unexported net/http, which means borrowing it will be non-trivial.
I suppose I could do something ugly like inspect and modify the output of Request.Write,
but that feels ugly and fragile.
It is definitely a minor issue, but it seems reasonable for other auto-added headers to
behave like the Date header in this regard. Evidence that the Date header behavior is
right is that I didn't consult documentation -- I just tried it, and it worked exactly
as I expected.
Filed issue https://golang.org/issue/5953 about the double
Content-Type headers.

Comment 4:

I think you can use Request.Write, as the Content-Type header is added by the
ResponseWriter's content sniffing code. Hijacking should bypass that.

Comment 5:

Dave: Good point, thanks. Connection: close is being returned for HTTP/1.0 requests.
Filed https://golang.org/issue/5955 for that.

Comment 6:

Confirmed: http://play.golang.org/p/zYXrsJVDDW

Comment 7:

We both meant Response.Write (oops), but you're correct; I just confirmed here as well.
It's a bit ugly, but hijacking it is! Thanks for the workaround, much appreciated.

Comment 8:

I'm pretty sure I meant Request.Write... I used it in my example code.
I'm going to mark this issue as WontFix because it's extremely rare that someone should
not want to write a Content-Type header, and the Hijacker provides a means of doing so.
Happy for others to argue otherwise.

Status changed to WontFix.

Comment 9:

But yeah, Response.Write is what you actually want. :-)

Comment 10:

As a side effect of fixing https://golang.org/issue/5953, setting
header["Content-Type"] = nil
now does what I originally expected. Given that there's a separate issue filed about the
Connection header, I'd say this can now be closed as fixed. :)

Comment 11:

Status changed to Fixed.