Add MergedAt field to PullRequestLinks

[Tatsuhiro Tsujikawa]

Please add MergedAt field to PullRequestLinks.
github upstream schema now has merged_at field:

      "pull_request": {
        "type": "object",
        "properties": {
          "merged_at": {
            "type": [
              "string",
              "null"
            ],
            "format": "date-time"
          },
          ...

Primary use case for this field is to know whether closed pull request is merged or not if API response returns Issues rather than PullRequests (e.g., search API).