out's status and in's params: fetch_merge: true don't interact well.

[nat-henderson]

Describe the bug
If you fetch a PR with

get: foo
params:
  fetch_merge: true

and then upload a status later with

put: foo
status: pending

You'll upload a status for the merge commit, which doesn't show up on the Github Pull Request page.

To Reproduce

Fragment:

resources:
  - name: foo
    type: github-pull-request
    source:
      repo: jtarchie/github-pullrequest-resource
      private_key: ((repo-key.private_key))
      access_token: ((github-account.password))

jobs:
  - name: make-problem
    plan:
      - get: foo
        params:
          fetch_merge: true
      - put: foo
        status: pending
        repo: foo

And then you make sure that the merge commit isn't empty, and bang, problem!

Expected behavior
I expect the "pending" status to show up on the Github PR page. But the merge commit isn't displayed on that page, so it doesn't. :)

I also expect to fix it myself, stand by for a pr.