Send a User-Agent of 'httpok' for health check requests

This helps identify the sender of the GET requests in web server logs.
Often there are multiple parties health checking the web server,
including a load balancer and httpok. It's nice to know which tool sent
which requests.
Add headers arg to TestConnection.request()

To better match the real HTTPConnection class (and friends).
Merge pull request #36 from djsmith42/master

Send a User-Agent of 'httpok' for health check requests