Disabled buffering gets enabled for images

I've found that `superagent.buffer(false)` invocation don't really work, here is an example:

```
const superagent = require('superagent')

superagent
.get('https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png')
.buffer(false)
.end((err, res) => {
console.log(res.buffered)
})
```
Output is `true`, which I believe is a bug.

I need to process the unbuffered response and then decide whether to pipe it to some stream, but I can't do that, because response stream is already consumed and written into `res.body`.

As a workaround I've set empty parser: `superagent.get().buffer(false).parse((res, cb) => cb())`

Reproduced at superagent@6.1.0