feat: Support FlushError method

[tie]

Introduced in golang/go@fd0c0db4a41 (go1.20) along with net/http.ResponseController.

Example:

package main
import (
"fmt""net/http""github.com/felixge/httpsnoop"
)
typeresponseWriterstruct {
http.ResponseWriter
}
func (w*responseWriter) Flush() {
fmt.Println("Flush called (but expected FlushError)")
}
func (w*responseWriter) FlushError() error {
fmt.Println("FlushError called (as expected)")
returnfmt.Errorf("flush error (as expected)")
}
funcmain() {
rw:=httpsnoop.Wrap(&responseWriter{}, httpsnoop.Hooks{})
rc:=http.NewResponseController(rw)
fmt.Println("Flush returned:", rc.Flush())
}

Expected output:

FlushError called (as expected)
Flush returned: flush error (as expected)

Actual output before this change:

Flush called (but expected FlushError)
Flush returned: <nil>

[tie]

Was merged as part of #33.