func NegotiateContentType(r *http.Request, offers []string, defaultOffer string) string {
	bestOffer := defaultOffer
	bestQ := -1.0
	bestWild := 3
	specs := header.ParseAccept(r.Header, "Accept")
	for _, offer := range offers {
		for _, spec := range specs {
			switch {
			case spec.Q == 0.0:
			case spec.Q < bestQ:
			case spec.Value == "*/*":
				if spec.Q > bestQ || bestWild > 2 {
					bestQ = spec.Q
					bestWild = 2
					bestOffer = offer
				}
			case strings.HasSuffix(spec.Value, "/*"):
				if strings.HasPrefix(offer, spec.Value[:len(spec.Value)-1]) &&
					(spec.Q > bestQ || bestWild > 1) {
					bestQ = spec.Q
					bestWild = 1
					bestOffer = offer
				}
			default:
				if spec.Value == offer &&
					(spec.Q > bestQ || bestWild > 0) {
					bestQ = spec.Q
					bestWild = 0
					bestOffer = offer
				}
			}
		}
	}
	return bestOffer
}
NegotiateContentType returns the best offered content type for the request's
Accept header. If two offers match with equal weight, then the more specific
offer is preferred.  For example, text/* trumps */*. If two offers match
with equal weight and specificity, then the offer earlier in the list is
preferred. If no offers match, then defaultOffer is returned.
package httputil
import (
	"github.com/golang/gddo/httputil/header"
	"net/http"
	"strings"
)
func NegotiateContentEncoding(r *http.Request, offers []string) string {
	bestOffer := "identity"
	bestQ := -1.0
	specs := header.ParseAccept(r.Header, "Accept-Encoding")
	for _, offer := range offers {
		for _, spec := range specs {
			if spec.Q > bestQ &&
				(spec.Value == "*" || spec.Value == offer) {
				bestQ = spec.Q
				bestOffer = offer
			}
		}
	}
	if bestQ == 0 {
		bestOffer = ""
	}
	return bestOffer
}