incompatability between FormSubmission and HTMLUnit post Jenkins 2.289

Several plugins have failing tests when using new versions of core (e.g. `mvn test -Djenkins.version=2.289`) and they seem to follow a simiular pattern

calling HTMLUnit's `click` to submit a form is resulting in Jenkins erroring with something like

  15.170 [id=94]        WARNING o.e.j.s.h.ContextHandler$Context#log: Error while serving http://localhost:64686/jenkins/job/testFolder/support/generateAndDownload
net.sf.json.JSONException: A JSONObject text must begin with '{' at character 1 of init
        at net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:499)
        at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:919)
...

which subsequently causes the test to fail.

On the surface this appears to be an incompatability between HTMLUnit and Jenkins bumping to the latest HTMLUnit seemed to make no difference.

So far observed the issue in

• support-core
• ldap
• ssh-slaves
• pipeline-input-step