Enable tests for proxy handler

There is some similar code in [server.js](https://github.com/wikimedia/restbase/blob/master/lib/server.js#L158). I'd like to move more towards raising [HTTPErrors](https://github.com/wikimedia/restbase/blob/master/lib/rbUtil.js#L250) for responses with a status >= 400, as that often cleans up the control flow & ensures that all error cases are handled.

As for detecting the precise error type: Using `isinstance HTTPError` would require access to the constructor, which is not always easy since some of these might come from preq & some might be generated inside of restbase. A slightly more precise alternative could be to use `err.name === HTTPError`.