Allow ArrayBuffer and related views as data

In order to push binary data under the form of `ArrayBuffer` and its related views (`Int8Array`, ...) one needs not to stringify those.

For the XHR adapter there is nothing to do as it natively supports [`ArrayBuffer` view](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#send%28%29) in `req.send()`.

Node's http adapter supports only string or `Buffer` thus a transformation to `Buffer` is required before setting content length header.