fs rc: fixes incorrect Content-Type in HTTP API - fixes rclone#7726

fs/rc/rcserver/rcserver.go

@@ -201,6 +201,7 @@ func writeError(path string, in rc.Params, w http.ResponseWriter, err error, sta
 	fs.Errorf(nil, "rc: %q: error: %v", path, err)
 	params, status := rc.Error(path, in, err, status)
 	w.WriteHeader(status)
+	w.Header().Set("Content-Type", "application/json")
 	err = rc.WriteJSON(w, params)
 	if err != nil {
 		// can't return the error at this point
@@ -294,6 +295,7 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	}
 
 	fs.Debugf(nil, "rc: %q: reply %+v: %v", path, out, err)
+	w.Header().Set("Content-Type", "application/json")
 	err = rc.WriteJSON(w, out)
 	if err != nil {
 		// can't return the error at this point - but have a go anyway

fs/rc/rcserver/rcserver_test.go

@@ -847,3 +847,22 @@ func TestServeModTime(t *testing.T) {
 	}}
 	testServer(t, tests, &opt)
 }
+
+func TestContentTypeJSON(t *testing.T) {
+	tests := []testRun{
+		{
+			Name:        "Check Content-Type for JSON response",
+			URL:         "rc/noop", // Assuming rc/noop returns JSON
+			Method:      "POST",
+			Body:        `{}`, // Empty JSON body
+			ContentType: "application/json",
+			Status:      http.StatusOK,
+			Expected:    "{}\n", // Expected JSON response
+			Headers: map[string]string{
+				"Content-Type": "application/json", // Adjusted to match actual behavior
+			},
+		},
+	}
+	opt := newTestOpt()
+	testServer(t, tests, &opt)
+}