Webhook Alert doesn't retain added HTTP header settings

[TheRiffRafi]

Kibana version:
7.8.0
Elasticsearch version:
7.8.0
Server OS version:
Cloud ESS
Browser version:
Any Browser
Browser OS version:
Any OS

Describe the bug:
Added HTTP header to Webhook connector is not persisted after saving the webhook connector.

Steps to reproduce:
1.Kibana > Stack Management > Alerts and Actions > Connectors > Create Webhook Connector > Add HTTP Header > Save
2. Edit newly created connector > Added HTTP Header not listed.

Expected behavior:
Editing the connector should show the added HTTP headers.

Errors in browser console (if relevant):
None

Any additional context:
I've been unable to check if the header is persisted somewhere else, but this is because I don't know where to look.

[pmuellr]

I'm able to repro this - it does appear to be saved with the action, just not displayed again when the action is edited again.

I verified that by listing the actions with a curl invocation:

$ curl https://user:password@kibana-host-name:port/api/actions
[
  {
    "id": "8e855aae-2dbe-473a-85b2-5ea9c5c32a84",
    "actionTypeId": ".webhook",
    "name": "sample webhook",
    "config": {
      "method": "post",
      "url": "http://example.com",
      "headers": {
        "foo": "bar"
      }
    },
    "isPreconfigured": false,
    "referencedByCount": 0
  },
  ...
]

The header I added was foo: bar, which did get saved with the action, but was not displayed when I edited the action again.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)