Expose superagent to perform it's configuration

Hello!
I'm working on simple NodeJS middleware. It's application/problem+json error handler.
I need to be able to configure superagent during test. I want to be able to do the following:
```js
var agent = request.agent(app);
agent.superagent.parse['application/problem+json'] = agent.superagent.parse['application/json'];
```