Add missing application/x-www-form-urlencoded header

Author: basil

github-pullrequest-plugin/src/main/webapp/scripts/featureButton.js

@@ -1,7 +1,9 @@
 function callFeature(button, answerPlaceId, parameters) {
     fetch(button.action, {
         method: "post",
-        headers: crumb.wrap({}),
+        headers: crumb.wrap({
+          "Content-Type": "application/x-www-form-urlencoded",
+        }),
         body: new URLSearchParams(parameters),
     }).then(rsp => {
         rsp.text().then((responseText) => {