support "Accept-Encoding" http header

exactly to support compress http response body with gzip. This feature is desired when get operation is frequent and value is large
```go
ww := io.Writer(w)
if w.Header().Get("Content-Eocoding") == "gzip" {
    gw := gzip.NewWriter(w)
    defer gw.Close()
    ww = gw
}
```