Allow user to define keepalive time interval.

[csdodd]

The keepalive interval is hardcoded to 55 seconds. This is the same as the connection timeout on Heroku (https://devcenter.heroku.com/articles/request-timeout#long-polling-and-streaming-responses).

This means that connections sometimes timeout unnecessarily because the keepalive intervals are too large. This PR allows a developer to set their own keepalive intervals.