fix(status-notifier): use UTF-8 charset in webhook POST body

[shaileshpadave]

What

StringEntity(data) defaults to ISO-8859-1 on httpclient 4.5.x. This causes Content-Length to be computed from the ISO-8859-1 byte count, which disagrees with the actual UTF-8 body length whenever the payload contains a non-ASCII character (e.g. en dash –, non-breaking space). Strict HTTP receivers reject these POSTs.

Fix: pass ContentType.APPLICATION_JSON (UTF-8) to StringEntity.

Fixes #1399

How to verify

1. Configure status-notifier with a webhook receiver that validates Content-Length:

   conductor.workflow-status-listener.type=workflow_publisher
   conductor.status-notifier.notification.url=http://<receiver>
   conductor.status-notifier.notification.endpointWorkflow=events/workflow
   conductor.status-notifier.notification.subscribedWorkflowStatuses=COMPLETED
   

2. Run a workflow whose output contains a non-ASCII character, e.g. {"note": "in–network"} (en dash U+2013).
3. Before fix: receiver rejects with Content-Length mismatch. After fix: notification delivers successfully.