Superagent now supports subclassing

[kornelski]

FYI, superagent now supports replacing superagent.Request with another class, like this:

https://github.com/visionmedia/superagent/blob/b5af0b49741b68f7b2504036466bf2c7bc2f56a4/test/use.js#L51,L61

so you could use as an alternative way of patching it.