Merging into a pull request's source branch does not trigger a pullrequest:updated webhook

If you have a setup where you have a webhook which should trigger on a pull request, but having only the PR updated or PR created events added to the PR, no webhook will trigger on PR merge. The webhook can only be triggered by adding repo:push.

Scenario:

1. Create a webhook with trigger: PR updated, PR Created
2. create a PR from branch Source to destination -> this will trigger a webhook
3. now make a commit to the Source Branch -> this will trigger a webhook for PR updated
4. create a branch Feature from Source branch while the PR is still open -> make changes to files -> commit those to Feature branch
5. Create a PR from Feature Branch to -> Source Branch -> Merge it
6. Event in Step 5 - will add a new commit to Source Branch thus updating the PR since the new commit is added to the already open PR from Source to destination - however this event will not trigger a PR updated webhook

The webhook will only be fired if repo:push event is listed as one of the triggers.

Expected behavior: Since the open PR is updated with a new commit(PR's merge commit), it should trigger a "pullrequest:updated" webhook event