Handle keep-alive requests

**WIP**

This adds handling of keep-alive requests to Sanford. If a host is configured to expect keep-alive requests and the protocol raises a `EndOfFileError` then Sanford assumes it's a keep-alive request and returns a 200.

@kellyredding - What do you think of this for handling keep-alive requests? The only thing I don't like is the condition to avoid calling the user's error proc. Alternatively, we could probably just add a default response for the `EndOfFileError` (200) and still call the user's error proc. In this case, it is on the user to handle an `EndOfFileError`. Maybe my real question is how far do we want to go with this in Sanford?