Option for immediate header flush

[Bobochka]

In case of body stream if first body bytes generation takes long this adds an option of "faster response".

Will add tests if general idea is ok.

[erikdubbelboer]

I'm wondering if this should be two functions of if it should just be an exported field named Response.FlushHeaders bool. What do you think @kirillDanshin ?

[Bobochka]

@erikdubbelboer @kirillDanshin Exported field and added tests

[kirillDanshin]

LGTM, but needs some production testing before merge, I think

[Bobochka]

@kirillDanshin any action from my side? We're already running a fork in prod that always flushes before body.

[erikdubbelboer]

Thanks. We don't need any more testing seeing as this doesn't change anything if you don't set the value to true. And even then, just calling w.Flush() is not something complicated.