Add content-type header to oauth2 request

It seems like the twitter api is now [enforcing a content type](https://dev.twitter.com/oauth/application-only) (see step 2) on the oauth2 post.

This fixes three failing tests:
```
    #followerCount
      1) should get the # of followers a user has
      ✓ should fail on an invalid username 
    #urlTweets
      ✓ should get the # of tweets a url has 
      ✓ should fail on an invalid url 
    #latestTweets
      2) should get the latest tweets from a user
    #latestHashtagTweets
      3) should get the latest tweets by hashtag
```

Was thinking that we perhaps want to merge headers in https://github.com/sydneystockholm/follower/blob/master/lib/request.js#L43 rather than replacing them?