feat: Allow buffers as mock response data

[runeh]

With this change, it's possible to return binary payloads from an intercepted call:

const mockImage = readFileSync('/path/to/image/png')
mockPool.intercept({ path: '/foo', method: 'GET' }).reply(200, mockImage)

I haven't bothered with docs yet. I'll update them if there's interest in merging this.

[mcollina]

Amazing, could you add it to the docs?

[runeh]

I added some docs. Turned out to be very little, but I think it covers the change reasonably well.

[mcollina]

lgtm