Fix issues with labels API

[sarina]

Follows on from #9 and fixes issues raised in code review.

[sarina]

@singingwolfboy : I deployed this and it seems to work (!!) - see https://github.com/edx/edx-platform/pull/4788

however I'm wondering why I'm seeing this the logs:

2014-10-08T00:56:38.869884+00:00 heroku[router]: at=info method=POST 
 path="/jira/issue/updated?user_id=sarina&user_key=sarina" host=openedx-webhooks.herokuapp.com
 request_id=d683b8d1-4ac5-47a8-9ed4-449a180819d4 fwd="10.85.107.3,165.254.226.247" dyno=web.1
 connect=1ms service=481ms status=200 bytes=345

2014-10-08T00:56:39.370548+00:00 heroku[router]: at=info method=POST path="/github/pr"
 host=openedx-webhooks.herokuapp.com request_id=cfd71405-0e99-4bdd-8932-d0e4c394ea30
 fwd="192.30.252.42" dyno=web.1 connect=1ms service=9ms status=400 bytes=199

2014-10-08T00:56:39.368106+00:00 app[web.1]: Received labeled event on PR #4788 against
 edx/edx-platform, don't know how to handle it