Adds X-Github-Event header

Fixes https://issues.jenkins-ci.org/browse/JENKINS-23662, https://issues.jenkins-ci.org/browse/JENKINS-23661
Breaking change introduced https://github.com/jenkinsci/github-plugin/commit/bcc3bc1ca704b2be3f1475b616fa1b209986a1fd
Merge pull request #888 from jimgolfgti/jenkins-github-v1.9-patch

Adds missing X-Github-Event header