func Gzip() vodka.MiddlewareFunc {
	return GzipWithConfig(DefaultGzipConfig)
}
Gzip returns a middleware which compresses HTTP response using gzip compression
scheme.
package middleware
import (
	"compress/gzip"
	"io"
	"io/ioutil"
	"net/http"
	"strings"
	"sync"
	"github.com/insionng/vodka"
	"github.com/insionng/vodka/engine"
)
type (
	GzipConfig struct {
		Skipper Skipper
		Level int `json:"level"`
	}
	gzipResponseWriter struct {
		engine.Response
		io.Writer
	}
)
var (
	DefaultGzipConfig = GzipConfig{
		Skipper: defaultSkipper,
		Level:   -1,
	}
)