Fix for new submit() method (PR rjsf-team#1058) also submitting the H…

…TML form

..., navigating away from current page at least in Firefox. Reason:
dispatched event was not cancelable, so preventDefault in onSubmit
couldn't cancel it.

Links:

* <https://stackoverflow.com/a/40916998>
* <https://developer.mozilla.org/en-US/docs/Web/API/Event/cancelable>

src/components/Form.js

@@ -189,7 +189,7 @@ export default class Form extends Component {
 
   submit() {
     if (this.formElement) {
-      this.formElement.dispatchEvent(new Event("submit"));
+      this.formElement.dispatchEvent(new Event("submit", { cancelable: true }));
     }
   }