Replace the shouldGzip impl w/ httputils.Negotiate

Previously the ParseAccept method from
github.com/golang/gddo/httputil/header was used in (*handler).shouldGzip.
It's not clear this method was meant to be exported and widely used,
and it's use in shouldGzip did not account for case-insensitivity.

Instead it's replaced by Negotiate added to github.com/tmthrgd/httputils
in tmthrgd/httputils@2983d2a.

gzip.go

@@ -10,7 +10,6 @@ import (
 	"net/http"
 	"sync"
 
-	"github.com/golang/gddo/httputil/header"
 	"github.com/tmthrgd/httputils"
 )
 
@@ -332,13 +331,8 @@ func (h *handler) shouldGzip(r *http.Request) bool {
 		return true
 	}
 
-	for _, spec := range header.ParseAccept(r.Header, "Accept-Encoding") {
-		if spec.Value == "gzip" && spec.Q > 0 {
-			return true
-		}
-	}
-
-	return false
+	match := httputils.Negotiate(r.Header, "Accept-Encoding", "gzip")
+	return match == "gzip"
 }
 
 func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {

gzip_test.go

@@ -51,6 +51,22 @@ func TestGzipHandler(t *testing.T) {
 	assert.Equal(t, gzipStrLevel(testBody, DefaultCompression), resp2.Body.Bytes())
 }
 
+func TestGzipHandlerAcceptEncodingCaseInsensitive(t *testing.T) {
+	// This just exists to provide something for GzipHandler to wrap.
+	handler := newTestHandler(testBody)
+
+	req := httptest.NewRequest(http.MethodGet, "/whatever", nil)
+	req.Header.Set("Accept-Encoding", "GZIP")
+	resp := httptest.NewRecorder()
+	handler.ServeHTTP(resp, req)
+	res := resp.Result()
+
+	assert.Equal(t, http.StatusOK, res.StatusCode)
+	assert.Equal(t, "gzip", res.Header.Get("Content-Encoding"))
+	assert.Equal(t, "Accept-Encoding", res.Header.Get("Vary"))
+	assert.Equal(t, gzipStrLevel(testBody, DefaultCompression), resp.Body.Bytes())
+}
+
 func TestGzipLevelHandler(t *testing.T) {
 	for lvl := BestSpeed; lvl <= BestCompression; lvl++ {
 		req := httptest.NewRequest(http.MethodGet, "/whatever", nil)