Fix "closers" of github issues/PRs in our webhook.

When folks closed issues or pull requests on github, we were
using the wrong field from the github payload.  Now we
correctly use the "sender" as the person doing the action.

(imported from commit 82989ab19b32f8e3f0bbff9b305a7cb2673d99e9)

zerver/tests.py

@@ -5301,7 +5301,7 @@ class GithubHookTests(AuthedTestCase):
     def test_pull_request_closed(self):
         self.basic_test('pull_request_closed', 'commits',
                         "zulip-test: pull request 7: Counting is hard.",
-                        "lfaraone closed [pull request 7](https://github.com/zbenjamin/zulip-test/pull/7)")
+                        "zbenjamin closed [pull request 7](https://github.com/zbenjamin/zulip-test/pull/7)")
 
     def test_pull_request_comment(self):
         self.basic_test('pull_request_comment', 'commits',

zerver/views/webhooks.py

@@ -27,7 +27,7 @@ def github_generic_subject(noun, topic_focus, blob):
 def github_generic_content(noun, payload, blob):
     # issue and pull_request objects have the same fields we're interested in
     content = ("%s %s [%s %s](%s)"
-               % (blob['user']['login'],
+               % (payload['sender']['login'],
                   payload['action'],
                   noun,
                   blob['number'],