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 in req.send().

Node's http adapter supports only string or Buffer thus a
transformation to Buffer is required before setting content length
header.
Merge pull request #10 from mathbruyen/arraybuffer

Allow ArrayBuffer and related views as data