【bug】snappy 未注册到 REM registry，compress=true 时崩溃

[winezer0]

snappy 未注册到 REM registry，compress=true 时崩溃

修复: 在 snappy.go 顶部添加 init()：

func init() {
core.WrapperRegister("snappy", func(r io.Reader, w io.Writer, opt map[string]string) (core.Wrapper, error) {
return NewSnappyWrapper(r, w, opt), nil
})
}