Drop accept-encoding header when supported (#157)

* Verify that supported Accept-Encoding header are dropped

* Drop accept-encoding header when supported

This prevents following handlers from encoding the request again since
we dropped the intention to do this.

Fixes #153

* Do not use subtests to be compatible with Go <1.7

compress.go

@@ -80,6 +80,7 @@ func CompressHandlerLevel(h http.Handler, level int) http.Handler {
 			switch strings.TrimSpace(enc) {
 			case "gzip":
 				w.Header().Set("Content-Encoding", "gzip")
+				r.Header.Del("Accept-Encoding")
 				w.Header().Add("Vary", "Accept-Encoding")
 
 				gw, _ := gzip.NewWriterLevel(w, level)
@@ -111,6 +112,7 @@ func CompressHandlerLevel(h http.Handler, level int) http.Handler {
 				break L
 			case "deflate":
 				w.Header().Set("Content-Encoding", "deflate")
+				r.Header.Del("Accept-Encoding")
 				w.Header().Add("Vary", "Accept-Encoding")
 
 				fw, _ := flate.NewWriter(w, level)

compress_test.go

@@ -49,6 +49,71 @@ func TestCompressHandlerNoCompression(t *testing.T) {
 	}
 }
 
+func TestAcceptEncodingIsDropped(t *testing.T) {
+	tCases := []struct {
+		name,
+		compression,
+		expect string
+		isPresent bool
+	}{
+		{
+			"accept-encoding-gzip",
+			"gzip",
+			"",
+			false,
+		},
+		{
+			"accept-encoding-deflate",
+			"deflate",
+			"",
+			false,
+		},
+		{
+			"accept-encoding-gzip,deflate",
+			"gzip,deflate",
+			"",
+			false,
+		},
+		{
+			"accept-encoding-gzip,deflate,something",
+			"gzip,deflate,something",
+			"",
+			false,
+		},
+		{
+			"accept-encoding-unknown",
+			"unknown",
+			"unknown",
+			true,
+		},
+	}
+
+	for _, tCase := range tCases {
+		ch := CompressHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			acceptEnc := r.Header.Get("Accept-Encoding")
+			if acceptEnc == "" && tCase.isPresent {
+				t.Fatalf("%s: expected 'Accept-Encoding' header to be present but was not", tCase.name)
+			}
+			if acceptEnc != "" {
+				if !tCase.isPresent {
+					t.Fatalf("%s: expected 'Accept-Encoding' header to be dropped but was still present having value %q", tCase.name, acceptEnc)
+				}
+				if acceptEnc != tCase.expect {
+					t.Fatalf("%s: expected 'Accept-Encoding' to be %q but was %q", tCase.name, tCase.expect, acceptEnc)
+				}
+			}
+		}))
+
+		w := httptest.NewRecorder()
+		ch.ServeHTTP(w, &http.Request{
+			Method: "GET",
+			Header: http.Header{
+				"Accept-Encoding": []string{tCase.compression},
+			},
+		})
+	}
+}
+
 func TestCompressHandlerGzip(t *testing.T) {
 	w := httptest.NewRecorder()
 	compressedRequest(w, "gzip")