Add trailer support

[ichxxx]

This is in reference to #1078 , #210 .
This will allow FastHTTP to support trailer for chunked transfers.

Write trailer

For example:

resp := &Response{}
resp.Header.Set("Trailer", "Foo")
// write body
resp.Header.Set("Foo", "bar")

req := &Request{}
req.Header.SetTrailer("Foo")
// write body
req.Header.Set("Foo", "bar")

Header Foo will send after the chunked body.

Read trailer

For example:

func(ctx *RequestCtx) {
    ioutil.ReadAll(ctx.RequestBodyStream())
    ctx.Request.Header.VisitAllTrailer(func(key []byte) {
        trailer := ctx.Request.Header.PeekBytes(key)
        // do something
    })
}

Trailer can be found by inspecting Header after reading all chunked body.

[erikdubbelboer]

Lint is failing: https://github.com/valyala/fasthttp/runs/4310069291?check_suite_focus=true

Can you add some clear documentation that trailers are only supported with chunked bodies?

[ichxxx]

Lint is failing: https://github.com/valyala/fasthttp/runs/4310069291?check_suite_focus=true

Can you add some clear documentation that trailers are only supported with chunked bodies?

Sure.

[ichxxx]

Seems the tests are a bit flaky.

[erikdubbelboer]

Seems the tests are a bit flaky.

Yeah they are. You can ignore that.

[erikdubbelboer]

Sorry for asking for so many changes. I'm super happy that you wrote this code. But it's also quite a lot and important to get right I think.

[ichxxx]

This is not a problem at all. It's always better to be strict with code. I have also learned a lot from this pr.

[erikdubbelboer]

Thanks!