fix transport http headers

pkg/cfg/transport.go

@@ -53,10 +53,7 @@ func (t *DebugTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		contentline = buf.String()
 	}
 
-	if req.Header.Get("Accept") != "" {
+	req = fixRequestHeaders(req)
-		req.Header.Del("Accept")
-		req.Header.Add("Accept", "application/json")
-	}
 
 	slog.Info("req", "host", req.URL.Host, "uri", req.URL.Path,
 		"body", content, "bodyline", contentline,
@@ -66,20 +63,31 @@ func (t *DebugTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	return t.Transport.RoundTrip(req)
 }
 
-// Fixes https://codeberg.org/scip/esctl/issues/79:
-// the API client has this Accept header hardcoded everywhere:
-// req.Header.Set("Accept", "application/vnd.elasticsearch+json;compatible-with=9")
-// While this works pretty well with ES9, it doesn't with ES8. So, we replace
-// this header with a new one without the compatibility part.
 type CompatibilityTransport struct {
 	Transport http.RoundTripper
 }
 
 func (t *CompatibilityTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req = fixRequestHeaders(req)
+
+	return t.Transport.RoundTrip(req)
+}
+
+// Fixes https://codeberg.org/scip/esctl/issues/79:
+// the API client has this Accept header hardcoded everywhere:
+// req.Header.Set("Accept", "application/vnd.elasticsearch+json;compatible-with=9")
+// While this works pretty well with ES9, it doesn't with ES8. So, we replace
+// this header with a new one without the compatibility part.
+func fixRequestHeaders(req *http.Request) *http.Request {
 	if req.Header.Get("Accept") != "" {
 		req.Header.Del("Accept")
 		req.Header.Add("Accept", "application/json")
 	}
 
-	return t.Transport.RoundTrip(req)
+	if req.Header.Get("Content-Type") != "" {
+		req.Header.Del("Content-Type")
+		req.Header.Add("Content-Type", "application/json")
+	}
+
+	return req
 }