Fix another issue with Content-Type detection.

If the response from upstream had Content-Encoding: gzip, but no
Content-Type, the Content-Type was being sniffed as application/x-gzip.
Decompress the content to fix the issue.

proxy.go

@@ -339,6 +339,17 @@ func (h proxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	removeHopByHopHeaders(resp.Header)
 
+	// Yet another workaround for https://github.com/golang/go/issues/31753
+	if resp.Header.Get("Content-Type") == "" && resp.Header.Get("Content-Encoding") == "gzip" && r.Method != "HEAD" {
+		gzr, err := gzip.NewReader(resp.Body)
+		if err != nil {
+			log.Printf("Error creating gzip reader for %v: %v", r.URL, err)
+		} else {
+			resp.Body = gzr
+			resp.Header.Del("Content-Encoding")
+		}
+	}
+
 	respACLs := conf.ACLs.responseACLs(resp)
 	acls := unionACLSets(reqACLs, respACLs)