Fix PubSubHubbub Service Hooks support (currently broken for Octokit >= 1.19) 

Refs issue #202

PubSubHubbub needs url encoded requests which have been removed from Octokit on V 1.19

This PR restores them and also adds assertions to ensure these methods calls trigger requests with the correct Content-type header (```application/x-www-form-urlencoded```).

Thanks @sigmavirus24 for hints about what causes that issue.