common: arrays don't serialize properly in post

[josephjclark]

Check this:

post('www', { body: ['x'] })

This will throw an error like:

Serializing the body works.

What's happening is that in the common request helper, we're trying to decide whether to serialize an incoming thing to a JSON string. And if the thing is an array then we basically fail because we don't recognise it.

There's a couple of tests in the code against Iterators and I can't remember why. Are they trying to detect steams or generators or something weird?