Auto add 'Vary' header after compression

[AutumnSun1996]

Add config SetAddVaryHeaderForCompression to enable 'Vary: Accept-Encoding' header when compression is used.

Added a config variable addVaryHeaderForCompress to control this feature, with setter SetAddVaryHeaderForCompression.
To remain backward compatible, it's false by default.
When enabled, 'Vary: Accept-Encoding' header will be added when compression is used.

fix #1583

[erikdubbelboer]

I have thought about it and I think it's better to remove SetAddVaryHeaderForCompression and always set the Vary header. We also always set Content-Encoding. Can you change that, then I can merge it. Thanks.

[AutumnSun1996]

Hi, the code is updated, please review again. Thanks.

[erikdubbelboer]

Sorry, but I'm thinking now it's probably better to use this. In case the user already set the Vary header before themselves.

	if v := resp.Header.Peek(HeaderVary); !bytes.Contains(v, strAcceptEncoding) {
		resp.Header.SetBytesV(HeaderVary, append(v, (","+HeaderAcceptEncoding)...))
	} else {
		resp.Header.Add(HeaderVary, HeaderAcceptEncoding)
	}

[AutumnSun1996]

See what you mean. But this leads to another question: should we also handle the case where the user has added 'Vary: Accept-Encoding'?

[erikdubbelboer]

That's exactly what my example does.

[AutumnSun1996]

oh sorry, I just missed the second check. going to update it now

[AutumnSun1996]

I created such a function to check and update the Header, instead of copy-paste. any more comments?

// AddVaryBytes add value to the 'Vary' header if it's not included
func (h *ResponseHeader) AddVaryBytes(value []byte) 

[erikdubbelboer]

This is such a specific function and our API surface is already huge. Lets not export this function.

[AutumnSun1996]

My thoughts was, it can also be used to add other values into 'Vary' header in application code.
Any way, it's ok to me to change it to 'addVaryBytes'

[erikdubbelboer]

Thanks!