Support Promise[Blob].

[dduponchel]

The old code didn't accept promises containing blobs which cause issues
when working with canvas. There are two ways to get a binary content from
a canvas: canvas.toDataURL() and canvas.toBlob(). Working with the
first one involves a substring to remove the data url part and a base64
decode. The other one ends up with a Promise[Blob], fixed by this
commit. Now, we can extract the binary content of a canvas like:

var p = new JSZip.external.Promise(function (resolve, reject) {
    canvas.toBlob(function (blob) {
        resolve(blob);
    });
});