For reproduce:

See attachment screen.PNG.

1. Create REST-task with parameter ContentType = "application/json;charset=UTF-8"
2. Save your diagramm
3. Open diagramm again
4. Open I/O properties of REST-task
5. You will see in field ContentType: "application/json;charset=UTF 8"

Workbench automatticaly replace symbol '-' to whitespace and after that, i can't sent data with non acii symbols data to external service.

The only solution I found, that's change generated bpmn-xml file by hands, and replace all 'UTF 8' strings to 'UTF-8'.