request.setHeader should fail if data has already been sent written to the client

[mg]

setHeader does nothing and says nothing if the response has started to be
written to the client. t.web definitely has this problem; I think t.web2
probably does too.

[jknight]

In web2 it would be very hard to do this by accident because of the API: 
you return a complete response. I doubt it's a good idea to change the 
behavior for t.web at this point; it'll almost certainly break something 
and has no real advantage.

[Jean-Paul Calderone]

Arguably, it only changes the way in which already-broken code fails, no?

[mg]

Yes, the code is already broken.

[jknight]

I don't think there's a good reason to change the web1 API like this.