Add the missing actions for PullRequestEventType

From the official Github API's docs:
```
Key	        Type        Description
action      string        The action that was performed. Can be one of “assigned”, “unassigned”, “labeled”, “unlabeled”, “opened”, “closed”, or “reopened”, or “synchronize”. If the action is “closed” and the merged key is false, the pull request was closed with unmerged commits. If the action is “closed” and the merged key is true, the pull request was merged.
```

so I just added the ones that were missing.