@razic I think this is causing problems with Ken's previous patch to support issues that are not associated with pull requests.

For example, when I rebased this change set on top of master, I'm getting these failures:

```
Failures:

  1) PuppetLabs::PullRequestApp posting an issue /event/github responds to /event/github
     Failure/Error: post route, params, env
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./lib/puppet_labs/pull_request.rb:59:in `load_data'
     # ./lib/puppet_labs/pull_request.rb:43:in `load_json'
     # ./lib/puppet_labs/pull_request.rb:31:in `initialize'
     # ./lib/puppet_labs/pull_request.rb:22:in `new'
     # ./lib/puppet_labs/pull_request.rb:22:in `from_json'
     # ./lib/puppet_labs/issue.rb:34:in `load_json'
     # ./lib/puppet_labs/issue.rb:22:in `initialize'
     # ./lib/puppet_labs/issue.rb:17:in `new'
     # ./lib/puppet_labs/issue.rb:17:in `from_json'
     # ./lib/puppet_labs/github_controller.rb:29:in `event_controller'
     # ./lib/puppet_labs/pull_request_app.rb:251:in `block in <class:PullRequestApp>'
     # ./spec/unit/puppet_labs/pull_request_app_spec.rb:41:in `block (4 levels) in <top (required)>'

Finished in 0.12543 seconds
2 examples, 1 failure

Failed examples:

rspec ./spec/unit/puppet_labs/pull_request_app_spec.rb:40 # PuppetLabs::PullRequestApp posting an issue /event/github responds to /event/github
```

I think this change set needs to take the scenario where an issue is being handled and that issue is not associated with a pull request.  The exception is because `sender` is nil after executing `sender = data['sender'] || data['user']` in lib/puppet_labs/pull_request.rb:59