gzip: change ETag to weak ETag after gzip

According to https://tools.ietf.org/html/rfc7232#section-2.1
> Likewise, a validator is weak if it is shared by two or more
representations of a given resource at the same time, unless those
representations have identical representation data.  For example, if
the origin server sends the same validator for a representation with
a gzip content coding applied as it does for a representation with no
content coding, then that validator is weak.

Therefore, after gzip, we should change the original etag to weak etag.

caddyhttp/gzip/gzip.go

@@ -105,6 +105,10 @@ func (w *gzipResponseWriter) WriteHeader(code int) {
 	w.Header().Del("Content-Length")
 	w.Header().Set("Content-Encoding", "gzip")
 	w.Header().Add("Vary", "Accept-Encoding")
+	originalEtag := w.Header().Get("ETag")
+	if originalEtag != "" && !strings.HasPrefix(originalEtag, "W/") {
+		w.Header().Set("ETag", "W/"+originalEtag)
+	}
 	w.ResponseWriterWrapper.WriteHeader(code)
 	w.statusCodeWritten = true
 }

caddyhttp/gzip/gzip_test.go

@@ -38,6 +38,14 @@ func TestGzipHandler(t *testing.T) {
 			t.Error(err)
 		}
 		r.Header.Set("Accept-Encoding", "gzip")
+		w.Header().Set("ETag", `"2n9cd"`)
+		_, err = gz.ServeHTTP(w, r)
+		if err != nil {
+			t.Error(err)
+		}
+
+		// The second pass, test if the ETag is already weak
+		w.Header().Set("ETag", `W/"2n9cd"`)
 		_, err = gz.ServeHTTP(w, r)
 		if err != nil {
 			t.Error(err)
@@ -114,6 +122,10 @@ func nextFunc(shouldGzip bool) httpserver.Handler {
 			if w.Header().Get("Vary") != "Accept-Encoding" {
 				return 0, fmt.Errorf("Vary must be Accept-Encoding, found %v", w.Header().Get("Vary"))
 			}
+			etag := w.Header().Get("ETag")
+			if etag != "" && etag != `W/"2n9cd"` {
+				return 0, fmt.Errorf("ETag must be converted to weak Etag, found %v", w.Header().Get("ETag"))
+			}
 			if _, ok := w.(*gzipResponseWriter); !ok {
 				return 0, fmt.Errorf("ResponseWriter should be gzipResponseWriter, found %T", w)
 			}